When people are leaving work for the day, their co-workers will often say "O-tsukare sama desu" which translates literally to "you must be tired." The point is that you're recognizing your co-worker's hard work that day and thanking them for sticking it out with you. The person leaving can respond to this by saying "O-saki desu." This literally means "is earlier" and you say it to point out that you're the one leaving earlier, so you're complementing your co-worker's industriousness and apologizing for abandoning them at the office.
Somehow, though, when the actual occasion to use these phrases comes up, I draw a total mental blank on "O-saki desu." Today I tried twice; once it came out "O-sake" (alcohol) and the other time it was "O kami" (oh God). Either one could be an appropriate sentiment to express upon leaving work for the day, but not exactly what was expected.