Here is my shmups scandal!
Years ago i was running an informal score contest for a 90s shmup on a handheld system on a different forum. There was not much interest since this is an esoteric game in a euroshmup style, but eventually one other guy participates. We just shared screenshots of the point total at the end of the game to evidence our scores.
I had some very high legit scores but each time I'd post one, the other fellow would magically beat that score by like 5 or 10 percent more within a day or two. After a couple back and forths like this, I felt that it would not be possible for me (or another reasonable player) to actually score much higher in this particular game. I also felt like the other guy was maybe just simply editing his screenshots to change the number... So I started doing the same! I posted another score that was higher than what we'd had before. Once again, he comes back in a day or two with a score slightly higher... I post another cheat edit score, that in my opinion would probably be impossible to actually get in this particular game... He posts another score a day later that beats it by like 5 or 10 percent... By this point , our most recent scores are well above where we started when we posted our initial scores in the thread. I ended my score contest after that and declared him the winner. There was no definitive way to tell... but I'm pretty sure we both devolved into simply sending edited screenshots to each other

I was an expert player of this particular game, and even in a perfect no death run where you kill lots of enemies and do other things to try and maximize score, I don't think a legit score would hit the heights our fake? Scores were hitting and things were getting absurd.
I'll add that this contest was not related to any websites that track scores or speedruns on any games so I don't think our scores fed into any hall of fame recorded records or anything like that.