During a game, you usually have your character sheet sitting in front of you, so you can record stuff like lost hit points, used spells, and all the loot you've collected. It can become tempting, when the GM isn't looking, to also record things like improved ability scores, new skills, and better equipment.

Just make sure that if you do succumb to temptation that you also add balancing features such as reduced scores in other areas or new disadvantages and flaws. It looks much less suspicious if your character remains - technically, according to a strict, literal interpretation of the rules - balanced at the same power level as before.

On the other side of the GM's screen, if you're running a game and your players' characters suddenly start sprouting flaws like No Fashion Sense, Inability to Smell Garlic, or Susceptibility to Hangnails, then you should be keeping a closer eye on their scribbling.

Transcript

R2-D2: I spray the combat droids with used waste oil.
[SFX]: shplurt...
GM: They start slipping on the floor.
Obi-Wan: So... what is up with you and Jim?
Anakin: I'd really rather talk to him about it first.
Obi-Wan: Okay, fair enough. Sorry.
R2-D2: I ignite the oil with my rocket thrusters.
[SFX]: Foom!
GM: Your what? Let me see your character sheet.
GM: When did you get these? I don't remember them.
R2-D2: I added those when I was GMing that night.
Obi-Wan: I knew it!
R2-D2: But I totally took a bunch of new Flaws to balance it.
GM: Go on.
R2-D2: Lactose Intolerance.
Obi-Wan: What?!
R2-D2: Allergy to Bees. Taste Good to Dragons.
GM: Okay, that's fine. You can keep all those.
R2-D2: Really?
GM: Roll for Detect Sarcasm.