I am a lawer, and all my friends know it 
Mostly, this is because I play in a powergamer game and I run my own average game. I wouldn't rule lawer (at least as much) in a game that is all about story.
A few times a DM has claimed to use the rules to create a very difficult situation. Actually, he was misreading the rules, and I called him on it. In the end he said "well, I rule it this way."
In a former game, a player rule-lawer so hard I got pissed and blew his character to kingdom come. And in ShadowRun, there is no comming back. The player just didn't like what was happening, so he argued.
Bottom line for me: if you are going to break the rules, say so. If you are going to use the rules, use them correctly. If you are changing the rules, tell me before it would come up (or come up again).

Mostly, this is because I play in a powergamer game and I run my own average game. I wouldn't rule lawer (at least as much) in a game that is all about story.
A few times a DM has claimed to use the rules to create a very difficult situation. Actually, he was misreading the rules, and I called him on it. In the end he said "well, I rule it this way."
In a former game, a player rule-lawer so hard I got pissed and blew his character to kingdom come. And in ShadowRun, there is no comming back. The player just didn't like what was happening, so he argued.
Bottom line for me: if you are going to break the rules, say so. If you are going to use the rules, use them correctly. If you are changing the rules, tell me before it would come up (or come up again).