My favorite grep mistake is actually from a related tool pgrep and pkill.

  pgrep foo -> finds things running matching foo
  pkill foo -> kills things running matching foo

except every year or so, I do something like this

  $ pkill foo
  $ echo nothing happened?
  $ pkill -9 foo
  $ echo nothing happened still? huh?
  $ echo ok, let's run this in verbose mode..
  $ pkill -9 foo -v 

but.. -v isn't verbose. since pkill is part of pgrep, and pgrep is like grep, -v is 'Reverse'.