R5-D4 was the little astromech droid that Uncle Owen and Luke Skywalker were going to buy in Star Wars: A New Hope. But the droid was defective, and blew a circut.

"Unlce Owen, this R2 unit has a bad motivator," said Luke. (actually, Luke, that's an R5 unit...not that anyone excpet die hard Star Wars geeks care)

This prompted C-3PO to suggest to Luke that they instead purchase R2-D2, which they did, and the rest is history.

Footnote: some lame Darkhorse Comic wrote a story about R5-D4, calling him Skippy the Jedi Droid. In the story, it claims that R5-D4 had a high concentration of midichlorians in the oil greasing his parts, thus giving him Jedi powers. However, he foresaw the future, and realized that it was R2-D2's destiny to deliver the Death Star Plans in order to save the Universe. So R5-D4 purposely blew his motivator so that R2 could be purchased instead. I'm not making this up....