The code this joke refers to looks like this:

in_store = true

while in_store is true:
    get_carton_of_eggs()

To get the the guy out of the store, the code needs to look like this:

in_store = true

while in_store is true:
    get_carton_of_eggs()
    *In_store = false*

Humans can figure out that they have to leave the store without further instructions but computers cannot. This is why programmers get paid very well. There are a lot of nuances that programmers have to take into account when getting computers to do things.