When teachers curve grades, does it ever lower someone's grade?

[Voyager]

@Absolute

Yes. In the classic implementation of grading on a curve, the bottom X% of the class gets an F, regardless of how well they did in an absolute sense. If the entire class scores at or above 90%, the people who got exactly 90% would presumably get Fs, since they are technically at “the bottom of the class” (and maybe 91% and 92% as well, depending on the other scores).

This is one reason why very few professors actually grade on a curve.

I suspect any professor would give it up if they got such a weird distribution, and convinced themselves that their test wasn’t so botched that anyone with a pulse could get a 90.

When we graded on a curve when I was a TA the point was to force the absolute grades into the distribution we thought was right for the class. That’s not necessary of you have taught the class so many years that you know what people would get. If you make the test hard, why penalize your class. Almost all tests I took at MIT were graded on a curve, and many class averages were 50 - 60.
Typically we gave Fs in the first test, as an encouragement for those who couldn’t hack the work to drop the class. In later tests we only gave Fs if they were richly deserved, and not on a pre-determined formula.