The first one is the painful awareness that only the random-number generator saved me from having something rather unsavoury on the first line of today's puzzle.
The second one is that there can be multiple answers. The I and A in the middle column are interchangeable, but STALL/SHIFT is correct and STILL/SHAFT is incorrect.
I spent half an hour trying to fix an intermittent bug with the answer-checker I was working on before I realised there was no bug: I was checking the right answer some of the time and the wrong answer the rest of the time.
The answer-checker was necessary because I added something to move the letters around. I haven't uploaded it yet, though, as I haven't tested it for smartphones (on account of how I haven't implemented it for them yet).