In the 60's I worked three years for Chicago Union Station Mail Transfer Department, under the main post office in Chicago.

One day while unloading a boxcar of mail, I found a package laying on the ground beside the tracks. It had over $17(1960's dollars) of Air Mail - Special Delivery postage on it, and the postmark was three days old.

I heard about a new mail carrier who would step outside the building, empty his route bag into the Chicago river, and go home for the day. It took almost a month for him to get caught because the business offices on his route were complaining about not getting mail.

One of my co-workers on the parcel sorting line had some sticky name-&-address labels made. Everytime something interesting went down the line, he would whip out one of his labels and paste it over the label on the package.
He got caught because the mail carrier delivering to his house complained about the amount of work involved in delivering 100-200 packages a day!?

I read in the newspaper that when the main post office moved operations to a new building, they found over three tons of mail in the bottom of an elevator shaft. Some of it as much as thirty years old.