My favourite has to be all those "fat burning classes" gyms put on that are always absolutely packed with normal people sweating their dicks off doing 45 minutes of burpees and what not. Meanwhile there's absolutely jacked monsters just casually walking around the gym between their sets never doing the class.
Like the disconnect should be obvious. Go ask them what they did to get their physiques and why it's so different than what you're doing lol.
Also there's just plenty of trainers who truly have zero idea what they're doing but managed to get certified in some bullshit class. I have one friend who runs those types of classes and has looked the exact same, in not has gotten fatter, in the 6 years I've known her. She posts literally every day how hard she's working, what workout she's doing today, what supplements she's taking and when, etc. If you didn't have her Instagram you'd literally never know she works out. Because yeah, when you actually know what you're doing you see that she's doing a bunch of nonsense that just makes her feel tired, gets her sweaty, and burns calories. But doesn't actually ever make her stronger hence why she's never improved her physique.
Then come beach/cottage time every year she's shocked at my improved physique and wants to know what I'm doing (lifting a few times a week), eating (whatever fits my calories and macros), and taking (whey, creatine, and collagen) and says I'm so lucky to respond like that. Don't have the heart to tell her she's wasting her time and I'm not as she actually is super passionate about staying active and everything. What she's doing makes her happy, even if it's useless, though she'd be much happier if she just did a Google search to figure out what progressive overload is.
I'm more mad at the people who ever certified her in all these bullshit things in the first place instead of teaching her what actually improves one's body. A good amount of trainers are knowingly scam artists just trying to keep clients longer. A good amount are also unknowingly victims of scam artists themselves and just passing the scam down to other unknowing victims.