Here is my lesson learned for today.
1. Do not talk about others (even if it is just joking). In particular, do not suggest Celebrity Death Match-esque battles between your co-workers and your Client's co-workers.
2. If you must talk about others, do it verbally versus via email.
3. If you must talk about others and it must be via email, do not copy your Client. They might not find the Celebrity Death Match-esque idea funny.
4. If you must talk about others and it must be via email and you do, in fact, copy your Client, be prepared to feel totally embarrassed, have an apology ready and make sure to instate damage control.
Lesson Learned: No more funny business via email!