And as a corollary, flat out stealing ideas/code/whatever. If it's still just a startup, no one (that matters) may have every done a thorough search yet to be sure that the technology provided by the founders is original. Or course it won't survive scrutiny from the press or due diligence from a potential buyer or anything.
A friend of mine worked with a startup where there were three founders and they were all notorious for taking unearned credit. I think at least one was a pathological liar. As time when on, the true origins of everything they "contributed" came out. Depending on how good they were at bs'ing and raising money at least, a couple were were fired quickly and the last one took years to get rid of (a big enough fraud scandal finally went public).