When Gruber has all the lorries full of the gold and is celebrating with his men, one of them tells him that the chopper will be ready in 10 minutes (on the subtitles, you understand.) BUT, he actually says "zwanzig," which means twenty.
Nice catch. You're right; it should be "zehn" not "zwanzig". I probably would've caught it but the last time I saw it I couldn't speak German! :) Good job!