But alas, changing the wrap to off on that particular form did not help.

Amazing spotting, vintermann.

Can you submit a patch for this? If not, due to the excellent work you've 
done identifying the issue, others should be able to in due course.

It definitively has to do with the text. I changed diffsingle.txt to only
contain the word "before" and diffsingle_result.txt to only contain the word
"after", and when I made a patch of that and pasted it, it worked.

I haven't been able to set up a local site yet (it's 23:37 here) but I have a
guess to what the error is: I notice that the patch I get from the current file
has lines longer than 80 characters. The patch from the old files didn't. My
guess is the form hard-wraps (and thus breaks) the pasted patch.

The "diffing individual files" step under the "Using diff and patch" mission
appears to be broken. 

I downloaded the source from git, and found the two correct files
(diffsingle.txt and diffsingle_result.txt in mysite/missions/diffpatch/data).
Running 'diff -u diffsingle.txt diffsingle_result.txt' in that directory
provides a patch which the form does not accept. So the problem is definitively
not on our side (there are four people in the forums saying this test doesn't
work now).

I think it must have been the changes to the text on 2. may (commit
c392308096fa5513d61b0e68f1d792e1778ac821) that introduced this bug, but I
haven't figured out what causes it yet.

Interestingly, the form does give the proper error message if you enter a
garbled patch, or a reversed patch.