Saying way too much, for way too long and sharing way more than necessary.
The biggest pitfall to stumble into in an interview is talking too much. When nervous, most people ramble on…and on. Eventually they talk themselves into a corner and wind up exactly where they didn’t want to be: the job where they got laid off, the manager that didn’t like them, and the course they almost didn’t pass.
Avoid the trap by being concise. Answer the question. Smile. Ask a question, or wait for another one. If the interviewer wants to know more, (s)he’ll ask. Silence isn’t a bad thing in an interview. Going off topic can be.