His E won't hit anything, during a ranked game, it refused to hit anything, it wouldn't hit creeps, jungle monsters, dragon, baron, or champs.
Tried it in a custom game and it does the same thing.

How to repeat
Step1: Get rumble with level 1 E
Step2: Fire your harpoon at anything it should hit, and it'll pass over it.

does it happen in every game with rumble or just once?

which champions were in the game?

which items did you buy?

masteries? runes? etetc...

do you really think this bug report is going to help with anything?

Riot are aware of this bug. http://na.leagueoflegends.com/board/....php?t=3050229