Letting Venomoth pass Quiver Dance when your Choice Scarf user/fastmon is dead.
Venomoth is a deadly Baton Passer, and if it can successfully Baton Pass to Togekiss or Nidoking then it's probably gg right there. Nidoking's coverage is unparalleled, so unless you have a full-health Porygon2 or Snorlax on your team it's going to be tough to beat a +1/+1/+1 Nidoking. Nidoking's Special Defense boost ensures that you can't rely on a bulky Water to beat it, as a Nidoking at +1 Special Defense is not falling to a Scald or a Surf without prior damage.
Togekiss is even more deadly, as it will flat-out deny you from attacking it with Air Slash. If your team doesn't have a Pokemon with greater than 426 Speed, then there is a chance that Togekiss will outspeed you. At +1 Special Attack, its Air Slashes will be doing at least 40% to Pokemon that resist it, meaning that if you can't attack on one of two turns not even Raikou or Zapdos will beat this bird. In fact, since its already titanic Special Defense is boosted by Quiver Dance then it's going to take two Thunderbolts before falling over. If Togekiss gets behind a Substitute or acquires a second boost just click the forfeit button, as there's no chance that you'll beat it. Togekiss is not OHKO'd by many Physical attacks, is barely 2KO'd by any Special attacks, and can rip foes apart with a boosted Air Slash.
Fortunately, Choice Scarf users such as Darmanitan, Heracross, and Flygon can rip the two QD recipients apart, so make sure that you keep your Scarfer alive if you see a Venomoth and Nidoking/Togekiss on the same team.
Also for the love of all that is holy don't let your Scarfer take the Sleep Powder. x_x