A physicist, an engineer and a programmer were in a car...
A physicist, an engineer and a programmer were in a car driving over a steep alpine pass when the brakes failed. The car was getting faster and faster, they were struggling to get round the corners and once or twice only the feeble crash barrier saved them from crashing down the side of the mountain. They were sure they were all going to die, when suddenly they spotted an escape lane. They pulled into the escape lane, and came safely to a halt.
The physicist said "We need to model the friction in the brake pads and the resultant temperature rise, see if we can work out why they failed".
The engineer said "I think I've got a few spanners in the back. I'll take a look and see if I can work out what's wrong".
The programmer said "Why don't we get going again and see if it's reproducible?"
This joke is an occupational joke. The humor lies in what the programmer said. In the situation that they were in, it would take someone that was crazy to want to try it again and see if they could make the cars brakes fail again. The indication is that the programmer is crazy, and this is funny.
Another funny part (to a programmer) is the fact that this is exactly what a programmer does when their code has a bug. We try to reproduce it so that we can be sure that it is something that we will be able to fix.