A Real Tro(u/o)per
My hard-working nature and get-it-done attitude inspired a former boss to remark several times that I was a real trooper.
The correct answer was that it should be "a real trouper." But I think "trooper" is an "error" only in the sense of "if the person reading your resume is anal about this kind of stuff, they'll throw you on the reject pile for it." But it's quite likely that "trouper" is an error in the sense of "accurately representing the speaker's meaning."
The expression originated as an analogy to a member of an acting troupe, with their "the show must go on" ethos. But the alternate spelling is, I think, being eggcorned into acceptability. When most people who aren't usage mavens hear the expression, they interpret it as a metaphor for a military trooper. And that interpretation makes sense, since members of the military are also known for their perseverence, as they, well, "soldier on" in the face of adversity. And when such a person -- and I include myself in this group -- turns around and uses the expression at a future date, the military analogy rather than the theater one is what's in their mind. So in that sense, when I (and I assume most other people) utter that set of sounds, what I'm really saying is that someone is a "real trooper," not that they're a "real trouper." My expression just happens to sound the same as one that's spelled differently.