The first time I used the Paladin I went through nearly an entire match not knowing how to deploy the shield because I rarely melee. "Where is the shield button? WTH?!"
Getting frustrated by being unable to revive a Salarian engineer only to realize I had been frantically clicking on his decoy.
Spending an entire match turning Hawk Missile launcher on and off while saying "what does this button do?" (Now I spec out of it completely, so there.)
Being unable to use the Acolyte at all because I kept popping back into cover before it charged. "Why won't this gun shoot?! I'm getting my face blown off! Better get back in cover!"
Spending many matches randomly turning blade armor on and off because it was in the #2 spot instead of #1.
And many other stupid things. I've basically done them all. Still doing them, in fact.
Especially Blade Armor.