Mobile editor fails if a given wiki uses abusefilter. An example is plwiki. I couldn't reproduce it all the times, but half of the times when trying to edit the API returned an abusefilter message saying that I was removing a lot of text (even though I was removing a single word) and the edit failed. It seemed that the second attempt would usually succeed.
Unfortunately, as it is now, when this happens the editor will show a success message and close, but will not save anything, making the user lose their changes.
This is similar to bug 52047, however the fix is not the same.