I worked at a major pizza chain in high school. One of the assistant managers would use his access to update his time sheet so that his login time was 12 hours off from his real login time. So if he logged in at 4pm, he would update it to really be 4am. At first glance his clock in times would look correct, but he was stealing 12 hours of wages. This went on for months before he was caught and fired.
That was the biggest thing I saw. There was a TON of smaller theft in the form of drivers faking customer complaints so that the order was freed out, even though the driver had been paid cash for the order.
A of my acquaintances from my hometown worked at a large retailer through highschool. They would hide merchandise under skids in the outdoor garden center during their shift, then come back at night to recover it. They would stuff small expensive items (ie: iPods), into the advertisement trays at the front of shopping carts, then recover them once the carts were pushed out into the parking lot. They built a "fort" between two aisles in the back warehouse to take naps during their shifts.
I could give stories like this for a long time. They never got caught (to my knowledge).
Not all low-level employees are thieves. But more of them are then most people realize.
That was the biggest thing I saw. There was a TON of smaller theft in the form of drivers faking customer complaints so that the order was freed out, even though the driver had been paid cash for the order.