I can forgive people who know nothing about Bitcoin for saying dumb things so my pet hate is Bitcoin enthusiasts who bitch and moan about Fractional Reserve Banking.
The most recent examples are the "...MtGox... running a fractional reserve bank" variety
I assume some of this is said in jest but an FRB should be solvent. Illiquid perhaps, but not insolvent.
Other stupid things said by bitcoiners:
"It is impossible for something to be valued higher than all the bitcoins in existence"
"Bitcoin makes it impossible to have interest bearing investments"