I have a possible solution but I can't figure out how to post it here without spoiling it immediately. If you tell me it's OK, or if somebody tells me how to post text with a black background (so you have to select the text to see it), I'll post it.

Paste it to, I don't know, https://dpaste.org

Using dpaste is a great idea! But I just realized that I was reading the clues wrong -- I thought the gray tiles were the user's attempts, not clues that the game gave you as already correct. It does indeed look like a bug or it's using a very obscure word. Sorry!

Yeah that's a bug - should be impossible. Thank you for posting, kinda important that that doesn't happen. I will check it out.

Wish I could tell you exactly what I was doing at the time, but I don't remember :( I only remember I was entering guesses as fast as possible even while the slow return animations were going on, maybe it has something to do with that.

Seems like a bug to me. The first word of the last puzzle starts with the C, not the V.