from the “Thought I was doing my job” Department:

A man got a job as a night watchman at a factory. There had been a
lot of thefts by the workers on the night shift, and so every
morning when the night shift workers passed through his gate it was
his job to check their bags and pockets to make sure that nothing
was being stolen.

Things were going along very well the first night on the job until a
man pushing a wheelbarrow of newspaper came through his gate. Aha,
he thought, that man thinks he can cover up what he is stealing with
that newspaper. So he removed the paper only to find nothing. Still
he felt that the man was acting strangely, so he questioned him
about the paper. “I get a little extra money from newspapers I
recycle, so I go into the lunchroom and pick up all the ones people
have thrown away.”

The guard let him pass, but decided to keep a close eye on him. The
next night it was the same, and the night after that. Week after
week it went on. The same guy would push the wheelbarrow of
newspapers past the guard’s checkpoint. The guard would always check
and find nothing.

Then one night, about a year later, the guard reported for work only
to find a message had been left for him telling him to report to his
supervisor. He walked into the supervisor’s office and before he
could say a word, the boss said, “You’re fired!”

–”Fired?”– he asked in total surprise. –Why? What did I do?”

“It was your job to make sure that no one stole anything from this
plant and you have failed. So you’re fired.”

“Wait a minute, what do you mean failed. Nobody ever stole anything
from this place while I was on guard.”

“Oh, really,” the boss answered. “Then how do you account for the
fact that there are 365 wheelbarrows missing?”