In Star Wars Episode 4: A New Hope, C3PO says that his first job was programming binary load lifters.
THREEPIO Vaporators! Sir -- My first job was programming binary load lifter... very similar to your vaporators. You could say...
Since he had his memory wiped at the end of Star Wars Episode 3: The Revenge Of The Sith, we know that this may or may not be entirely true, as he only remembers events after the end of The Clone Wars.
Does C3PO even know that his memory has been wiped?
(Anything from C-Canon and up will be accepted here.)