Jumping guard. In addition to being extremely silly to watch, jumping guard is a really high-risk technique for the person having it done on them. There is not enough time to react safely, it's an easy way to get someone's knee blown out, and it's an embarrassment to the sport.

Twisters. There are exceptions to this (for pro-level athletes all subs are okay in my eyes), but there's no reason to be doing neck-cranking submissions to your training partner who's literally just trying to get a workout before going back to work. Also, it's just not a good way to make friends. Life is rough for "Twister guys".

Flying scissor takedowns. Also called "Kani Basami", this takedown is really dangerous. There are literally dozens of examples of people getting injured from this move. There is a safe way to do it that's actually legal in the IBJJF (where basically nothing is legal) but other than that, avoid this at all costs.

Slams/suplexes. If you don't know how to put someone down without altering their life, don't pick them up :)

Full-Nelson. If it's illegal in ADCC, it's probably not something you should do in training with people that you claim to like. It also doesn't really have practical application to BJJ besides hurting someone.

Anything that you don't know how to do. If you don't know what you're doing, there's a good chance you'll injure someone while doing it. Learn before you try and hurt people!