A zombie with it's legs blown off. It can only crawl after you, making it much less of a threat than if it were able to sprint. In the popular video game Call of Duty: Black Ops, the "Crawler technique," is utilized to give players more time to purchase guns, talk strategy, ect. (The round doesn't shift until every zombie has been killed therefore players have as much time as they please if they turn the last zombie into a "crawler.")
Yo I'm out of ammo! I need to go get some after this round!" "Someone get a crawler.