I recently noticed this question Wierd youtube logo today, which clearly has nothing to do with Ubuntu, however if it had turned out to be a virus or something that was specific to Ubuntu or something like that, then it would have been on-topic, but now that we know that it definitely wasn't, do we just leave this question or close it as off-topic?
Yes, of course.
For example, if, during the course of getting details from OP, it turned out that it was because OP was using CentOS instead of Ubuntu, it would still be off-topic.
The classic example would be problems that couldn't be reproduced - that's right there in the list of off-topic reasons. All of these started out as problems and became ... not-problems?