I had that from a Microsoft interviewer. Thought my code was O(n^2) because he was a C guy...whereas I was writing it in Java (something I had checked would be okay with both the recruiter and the interviewer). Querying the length of a string is an O(1) operation, not O(n), so while you could make the case it's suboptimal (since a function call per loop instead of just a variable lookup), it's not quadratic in behavior. And when he asked what I would do if a number overflowed and I said "...let the exception bubble up because based on the function you asked me to write there is nothing I can do cleanly within it" it was pretty clear the interview was over.

Good times.