Skelly covers a number of things we've all had clients swear to us are correct interpretations of the law.
Of course, there are things which are worse than these assertions. Usually, I can convince Client that his interpretation is wrong (although, more than once that has involved yelling matches). The worst problems I've had have been the guys who actually know enough of the law to get themselves in trouble.
For instance, clients who are used to the way the legal system works in the cities often come to the conclusion that even if there is absolutely no doubt as to their guilt doing things which will make the prosecutors' lives harder will cause a better deal to appear. Apparently, just saying the word "jury" in the cities gets you a lot better deal. It doesn't work that way in the suburbs or more rural communities. In the suburbs the prosecutors lick their lips at the possibility of a jury. They figure the more conservative juries will break in their direction and, since juries sentence in Virginia, that the client will get much more time then he would otherwise. It may crowd the docket a little but that is worth the extra punishment expected. In the rural courts they have so much time available that they just aren't crowded by a jury so asking for one has no real effect (except, again, to expose the client to a conservative jury).
But try to explain that to a client who has had three charges in the city knocked down to misdemeanors the day before his jury trial and who's seen other people get the same treatment. Some refuse to listen. And of course, when Client gets 3 1/2 years for an offense a judge would have given him 7 months on, it's my fault.