Mistakes are going to happen.
Especially when you first learn a new technique, and even more so when that new techniques is an unusual as is sprang. So I have put together a list of errors you may make and how to fix them as well as a list of when things don't look right and what error may have caused this.
Dropping a thread:
Dropping a threads means you passed over it and did not work with with the rest of the threads. Dropping a thread is easily done with a back thread and is an error that cannot go unfixed. Even if you pick the thread back up on a subsequent row, work will not return to normal.
How to spot: your back threads will be shifted over another thread and will pass behind three front threads rather than two. You will have an extra front thread at the end of the row. You can feel the dropped thread hanging loose if you run your hand across the back of the warp or see it if you turn your loom over.
How to fix: return to the row where the thread was dropped and rework from there.
In the above photo I have dropped a back thread on the 2nd row (below the blue line), the area where the back thread should have been picked up is inside the yellow circle. Note that the back threads on the 2nd row cross behind three front threads, but the back threads only cross behind one front thread on the 3rd row. Even though the back threads are back to the correct position (crossing behind two front threads) on the 4th row, I have two threads at the end that were not worked on the prior rows and are now joining the normal two threads that would be at the end of a shift row.
Crossing two threads:
This is a cosmetic only error and will not affect your work. This is a common error that happens if you warp is too loose, has uneven tension or if you recently had to remove already worked rows to correct another error. Be sure your threads are all laying side by and side and that you have appropriate tension on your warp before working a row.
How to spot: there will be a small bump where the threads crossed.
How to fix: if you choose to fix the crossed threads you will either need to remove the rows below it, or just remove the interlinkings below it, fix the error, then rework the interlinkings back the current row.
Although not a fatal error to your work, a crossed thread looks unsightly.
Working two threads as one:
This can happen with either front or back threads quite easily and is a common error. Essentially what happened is you accidently let two front threads drop as one, or picked up two back threads together. If not spotted and work continues correctly, the error will work itself by the next row of the same type. So if the error was made on a shift row, then the following basic row will be incorrect, but the row after that, the next shift row, work will return to normal.
How to spot: the interlinkings after the error will be incorrect in that either there will be no interlinks, just threads twisting around each other (if two front threads dropped) or work will have shifted over by another thread and the back threads will pass behind three front threads (if two back threads are picked up). You will also have the incorrect number of ending threads.
How to fix: you will need to take out all the interlinkings below and to the left of the error. Depending on your warp, you may just want to pick up the row where to the error occurred and remove all the rows below it then rework them after correcting the error.
Here I have dropped two front threads when I should have dropped one. The remaining row is not interlinking properly as a consequence. The next row is duplicating the same errors. The third row has returned to normal as will any remaining rows worked.
Turning the work upside down:
If you are working on a small portable loom and have no way of knowing the top from the bottom of your work, then you may accidentally pick up your loom upside down and continue working without noticing. When this happens the slant of your work will change. If you are not yet familiar with the concepts of S and Z twist interlinking, you may not realize what happened. Note that simply turning your loom around and working from the back, but with the top and bottom of the loom still in the original orientation, the work will not be effected. If you pick up your loom and the first two threads are sitting together behind the shed stick, you need to rotate your loom around. This is only apparent on a shift row. On a basic row it will be harder to tell your loom as been rotated.
How to spot: the direction that you threads are slanting will change. There will be a row where rather then interlinkings, the threads will simply cross each other. The path of a back thread will be to cross over two back threads then under one front thread. Normal interlinkings will resume on the following row.
How to fix: you will need to pickup the row where the change was made and take out all of the rows below it.
Notice the two threads at the beginning of the row that are sitting behind the shed stick. This is incorrect, the two threads should be sitting in front of the shed stick. If you tried to work this row in this orientation, it would be very difficult.
In addition to the obvious change in slant, notice that the ends of my warp are tied to the top dowel. I always tie my ends to my bottom dowel, so this would be the first clue that I had my loom upside down.
Warping an uneven number of threads:
You can warp up any number of threads you like, as long as there are the same number of threads behind the end dowel as in front of it. If you tie your yarn on to the bottom dowel, but then tie it off on the top dowel, then you will have an uneven number of threads between the back and front threads. This is a mistake that you will likely only ever make once, if that.
How to spot: when you get to the end of your row you will have the incorrect number of remaining threads, most likely three front threads, but it depends on what row you are on and which side has too many threads on it. If you already worked the first row where there were three threads, your subsequent rows will begin and end differently. On a shift row you will be left with just one front thread at the end, on a basic row you will be left with two front threads at the end. Because this is also what happens when you made an error in the middle of your warp, you should first check for errors in the row you just made. If no errors are found, check your warping or count how many front and back threads you started your work with.
How to fix: You can fix this at the first row where you have three front threads remaining by swapping one of the front threads to the back and interlinking it with the front threads. After that, your rows will begin and end differently, but otherwise the work will be normal. Or, you can untie one of your warp ends and pull it out of the work to the other end of the warp and then retie it to that dowel (your warp should now begin and end on the same dowel). You likely will not need to undo any rows, though it depends on how you handled the extra threads when you came across them.
How to Fix Errors
I have two videos on fixing errors if they are further back then the current row you are working on.
Errors by the Result
Below is a list of things you may notice in your sprang warp that tell you you made a mistake, and what types of mistakes may have made them.
Three front threads remaining at the end of a row:
1. Working two threads as one.
2. A warp with an uneven number of front and back threads.
3. Dropped back thread
One front thread remaining at the end of a shift row (where there should be two):
1. Working two front threads as one
2. A warp with an uneven number of front and back threads
Two front threads remaining at the end of a basic row (where there should be one):
1. Working two back threads as one
Back threads crossing behind more than two front threads:
1. Working two back threads as one
Back threads crossing behind only one front thread:
1. Working two front threads as one
2. Turning your loom upside down and continuing work
You have two front threads together at behind your shed stick at the beginning of your row:
1. You loom is upside down, turn it around so the two threads sitting together at the beginning of the row are in front of the shed stick
The slants of your threads have changed direction:
1. You turned your loom upside down and continued working
Although not a complete list of errors you might make in sprang, these are some of the most common ones. If I've missed any obvious errors that should be on this list, let me know!