Star Wars POTF - R5-D4

If this droid's motivator unit hadn't failed at just the right moment, the whole overthrow of the Empire may never have happened!

Owen Lars was about to buy R5-D4 from the Jawas on Tatooine when it malfunctioned, causing him to buy R2-D2 instead - and the rest is history.  There has been speculation that R5-D4 was actually sabotaged by R2-D2 in order for him to continue his mission to deliver the Death Star plans to Obi-Wan.  Whatever the truth, R5-D4 was eventually sold on to a Rebel spy and became an important member of the Rebellion as an intelligence gathering droid at Mos Eisley.