Not really a "great" quote, but it cracked me up.
"German made - American modification" - written on a picture of a knocked-out German Panzer.
One for the Engineers 
"Yessir - Ol' B Comp'ny broke another bridge buildin' record ..... a Kraut regiment is retreatin' across it." - Willie & Joe
Erwin
"Never tell the Platoon Sergeant you have nothing to do."
- Unknown Marine Recruit
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Ooh, I did once to my platoon leader. Fortunately it was my last day with the armed forces anyway. Our staff-sergeant meant I should have said that I'd have to tidy the LCT's cellar. 
Launcher Control Trailer