Running into people with Garuda/Titan/Relic weapons who don't know how how Chimera, or Hydra, or Garuda adds work. Running into people with Relic weapons who do not dodge Imminent Catastrophe ever. I auto-assume everyone is inept at Titan regardless of gearing level, but the short version is...

I try to ask if people are new when coming to content where -their- mistakes can kill everyone else. Otherwise, if they're silent, I assume they either know or will learn the hard way. That said, it is incredibly frustrating when you actually ask people if they know content simply because you wish to explain it to them, and they flat out lie to you.