Backspace causes the cursor to jump to the end of the replybox on mobile. Firefox/droid12
Say I type "I things I should asses" instead of "I think I should asses", I move the cursor to behind "things" and now I "should ass"
I've had this happen with Xenforo forums, Firefox, mobile, 'droid, but also only occasionally. I've found it less tedious (with longer posts) to compose it in Notes and paste it in, not that it's less hassle, complicating things, but I'm weird.