Infinitely many mathematicians walk into a bar

Infinitely many mathematicians walk into a bar. The first says, “I’ll have a beer.” The second says, “I’ll have half a beer.” The third says, “I’ll have a quarter of a beer.” Before anyone else can speak, the barman fills up exactly two glasses of beer and serves them. “Come on, now,” he says to the group, “You guys have got to learn your limits.”

This joke is a math joke, and also a "walks into a bar" joke. The joke here is that the sum of 1+1/2+1/4+1/8... to infinity is 2, and in order to know that you have to be able to calculate the limit of a series.

Normally to tell someone that they have to "learn their limits" means that they do not know what they are not capable of doing, so they need to figure that out and stop trying to do things that they are not able to do, but here the bartender might be telling the mathematicians to learn how to calculate limits.