I've had this happen quite a bit when investigating things, where I get the wrong idea from one step, and don't realize it until I've run with it for some time.

I've had this happen quite a bit when investigating things, where I get the wrong idea from one step, and don't realize it until I've run with it for some time. This is a normal occurrence and we do it all the time. In an interview setting, with added stress of being watched and judged, the problem is magnified. In interviews, I'll always throw in comments to help clarify if it looks like they're going off the rails, because letting them run on an incorrect assumption is only going to diminish your ability to judge their abilities. We all make mistaken assumptions all the time, about everything. The person who made no bad assumptions in an interview just got lucky.