1. A good rule of thumb is not to post anything on the internet that you don't mind having read or seen by your spouse, your mother, your boss, your attorney, and a whole room of third graders.
2. The internet is far less anonymous than you think. Even if you cover your tracks enough so it's impossible to tell who you are by your postings alone, all it takes it a court order presented to your internet service provider to release their records.
If you are concerned that posting something may be used against you in a court of law, don't post it in the first place. If you have legal questions about posting something, talk to your attorney as we can't give legal advice here.
One other thing I forgot to add is that what you post on the Internet can be forever. Even if you post something and then immediately delete it, there's no guarantee that a copy hasn't already been cached by Google, or that someone made a copy/screen grab of it.