They are commonly found in the drain valve. The drains are gravity drains, so no pump, they open and water dumps out. When the drain closes a bra wire can get stuck in the valve causing it to not shut all the way and cause a slow leak. This will cause your washer to lose water and have to fill mid cycle.
The other issue is they get stuck in the hose after the drain and catch lint and eventually get completely stopped up.
These are easy fixes that take a little time to learn but I clear them in 5 to 10 minutes most of the time.