I've been scanning this thread to see if it's been posted already but I can't seem to find it.
In an online race, if other cars make a pit stop, the moment a car stops at it's pit stall, other racers on the track will notice a brief, momentary freeze/stutter. While it may not seem like a big issue at first, and in fact, it's barely noticeable if you're not really watching out for it, if you happen to be racing closely with another car, it can result in calamity because the freezing results in some cars behaving as if they're stationary on track during that period. In one particular race at Suzuka (20 laps w/weather change), two players who had been racing literally neck and neck for 12 laps had their race end in ruins when somebody else pitted and it was as if one of the cars had hit the other dead on at 200 kmh. Both spun and stopped instantly, as if they had hit a hard wall. We all saw the same result in the replay and it was exactly at the moment when another car hit it's pit stall.
This was tested with different hosts on a variety of different tracks with the same results. It's clearly repeatable. And it's not just a momentary bit of lag--cars instantly freeze on the track for an instant. I'm just really surprised that nobody has mentioned this before.
If you race online in long/er/ish races with pit stops, make sure you're not close to another car if somebody pits.