Yesterday I was watching DeMusliM's stream and he had something like "Score: 154-2" written on the screen. But on the W3 booster overlay, his stats were 154-24. So at least 22 joinbugs resulting in a loss. Assuming he got the information about his 154 wins from the portrait screen, and assuming that he got as many wins as losses in joinbugs (seems to be around 50/50), there would also be something like 22 joinbugs in his wins. Total would be 44 joinbugs out of 178 games: ~25% of errors.
On my profile, I have at least 59 joinbugs recorded (stats in races that I never played). And I have other joinbugs "hidden" in the stats of my main race too. Assuming a similar rate, that would be ~75 joinbugs out of 365 games. So ~20% of failures whenever I press the "play" button.