Error messages that are totally misleading and give you no clue about what you did wrong and keeps you searching at the wrong places. As a recent example:
$ ab -k -c 100 -n 20000 https://server.name:1234 ab: invalid URL Usage: ab [options] [http[s]://]hostname[:port]/path
This took me quite a while to figure out why this is an invalid URL.
$ ab -k -c 100 -n 20000 https://server.name:1234/
ab needs the trailing slash.