"Timebug" happens when the in-game time is not equal to the real time. I first found this when looking at AlKoTho's Silverstone lap times in GT class during their career mode speedrun.

I decided to investigate further. I took a race three of us did (Springfield in GT class) and compared our first lap IGT to our first lap RTA. I timed the RTA by taking the first frame of the first lap (basically when the timer first goes from .00) then taking the first frame of the next lap. Here are the results:

AlKoTho (PC, 30 FPS) IGT: 1:38.94 RTA: 1:38.333

alchemistake (PC, 30 FPS) IGT: 1:42.62 RTA: 1:41.367

AdamLikeScars (PS2, 30 FPS) IGT: 1:42.26 RTA: 1:42.333