I've done this six times. It's really taxing on my job at CSX.
It has happened to me sex times... crap, seven, and the joke won't work anymore :(
There's also immortal "s \in X", I've seen it certainly more than once.
So the question that comes to mind is: What did he think he was there for?
You know, when cox is 0, then sex doesn't exist :P
I was expecting a Brian Cox joke =)
This is even worse in Sweden, because Cos(x) reads out as cow sex in Swedish. It cracks the students up every time.
Thinking about it in Danish.
cos(x) - cos x - cos "ex" - ko sex
What has been read cannot be unread.
True story: while being observed by staff teaching the efficient market hypothesis, my business professor wrote "semin" on the board instead of "semi-strong." It was tangibly awkward.
In an algebra class, the professor kept writing that the field K contains the field F adjoined with u, or: F(u) \subset K. Or even better, the group K contains the product of groups F and U.
Though not exactly a math class(math and music being related and all), in choir the director wanted to start at the measure with a letter F above it, so to start us he said, "One, Two, F'n go." Needless to say it took us quite a while to get back on track with a room full of high school boys.
Just yesterday, I was actually telling my roommate how I'm always doing this.
My teacher does this on purpose!
Experienced teachers all do this on purpose - after the fifteenth time you get over the embarassment and you've got to give students some light relief after an hour of trigonometry.
Or how about X is an element in set S and then accidentally switch the letters? Oops!
Reminds me of http://files.sharenator.com/sexisfun_Sex_Fun-s320x518-914-580.jpg :)
Entropy as a function of energy and position
My CS professor last term was writing code on the chalkboard and accidentally put 'pubic int foo()' instead of 'public int foo()'. He had no idea until a student pointed it out, then again the whole class didn't laugh so maybe we were mostly zoned out.