It is the second time it happens to me today, I take 5 minutes to understand a question, then 10 minutes for writing (and editing multiple times) a detailed answer or comment, I'm proud of me, and just after having finished writing my proof, suddenly, the OP deletes the question... hence my work is lost forever, and that's a little frustrating !
maybe it'd need a rule such as
- a question cannot be deleted during the 20 minutes after the last comment or the last answer ?
this would let the OP think to if he'd really want to delete, and it would be much less frustrating for the answer-er, as he'd probably won't notice that the question has been deleted..