Starting off with a pretty notorious glitch (if you're in the right circles anyway) - the MarisaB bug! In Touhou 10, Marisa's second shot type is coded incorrectly so that when her power is between 3.00 and 3.95, she deals 10 times as much damage to bosses as she's supposed to while unfocused. Normally, this wouldn't be that big of a deal because you spend most of the game at max power, but TH10 is a "bomb from power" game, where you have infinite bombs but bombing drops your power by 1.00 - so you can basically cheese the entire game by immediately pressing X as soon as your power reaches 4.00! The Touhou Community-Reliant Automatic Patcher program that English-speaking fans use to play the games with translated dialogue bakes a fix for the MarisaB bug into the translation patch, so a lot of western fans have only heard about this bug secondhand and never actually experienced it. But that hasn't stopped it from being notorious anyway. #glitchposting
Here's a demonstration of the glitch in action: https://www.youtube.com/watch?v=UHvdsFjW4l4 You can see how easily this trivializes every pattern in the game except for the handful of survival spells!
(( i struggled against the last phase for kanako for 10 long years and it was the marisab glitch that finally let me beat her. actually broke down crying afterwards, like ugly crying ))