We will start the list with one of the most obvious things that people have found inside their car tires, with nails often being the cause for a flat tire or slight punctures because nails are often left around on roads. Whether it is from construction sites or because people
have dropped them by accident, a nail or screw being found on the road isn't a major surprise and if a tire was to drive over one, the nail would certainly stick in.
This is one of the most common reasons for people getting flat tires, with drivers going over them without noticing most of the time due to how small they are. But the damage can certainly be significant.