I had a friend who performed blatant acts of rules bending/breaking and hid them out of a desire to protect his CharGen secrets. He was a big fan of "Secret Wars" style mass combats, creating characters of a given point base and having them brawl FNAR, an there was no need for a GM (no story), so we made the characters and went on an honor system. Eventually I'd find out what he was doing and show him that it wasn't legal, but until I found out, things were pretty rough. Here's the three worst offenses...
First, he thought that density increase added to your figured characteristics. Thinking this, he realized that one level of DI (old rules) was worth negative total points and he went nuts with it, making a character with 20 levels of DI. Before I realized what he was doing, this character found himself KO'd from a round of NNDs from a pack of agents. With 20 levels of DI, we couldn't get enough strength between all of the characters there to pick him up to carry him away from the scene.
Next, and this was his most abused effort, he discovered negative figured characteristics. Only one figured characteristic can be negative, but he neglected to remember this and started making characters with CON scores in the thousands. Almost overnight, his roster of characters got WAY more powerful. Eventually, I got him to fess up as to how he did it, showed him the rule, and that was the end of the problem.
Finally, I watched him roll his dice one-by-one and toss each successive die at any result that came up a 1 or 2 in order to change the result. As you can guess, this didn't last long either. Though it isn't a rule, it's pretty bad to see someone try to justify this...
For all the headaches caused by the above, there were still some good times, so I don't regret it. I just remember them as some of the worst examples of rules interpretation that I'd ever seen.