I’ve committed two cardinal sins of consulting: I was, for all intents and purposes, unreachable for several days and I have long lingering outstanding expenses.
I’ll save you, Dear Reader, from any details or explanations or excuses. Instead, I’ll use it as a launching point for composing a list of Consulting Sins.

1. Discussing the client in public
2. Posting on-line about the client, especially during client meetings.
3. Leaving one client’s name & references in a document or presentation for another client.
4. Abusing expense account and billable hours.
5. Not being reachable.
6. Letting expenses accumulate.
7. Failing to submit billable hours on-time.
8. Over promising and under delivering.
9. Booking yourself in two places at once.
10. Lack of preparation.
11. Don’t proof read, peer review, spell check, and grammar check things going in front of the client.
12. Overestimate the amount of time you have to deliver anything – you never have enough time.

I’m sure there are more. One colleague of mine would definitely include failing to carry a stain remover. Add your recommended cardinal consulting sins in the comments.