I think parking issues must cause quite a high percentage of neighbourhood disputes!
One of our neighbours has a "courtyard" big enough for 2 cars but their gate is broken and doesn't open so they don't use it and park outside of our place. The neighbours opposite could fit four cars easily on their yard, six at a push, but they also park their cars and vans outside our house. Another neighbour routinely takes up 2 spaces.
Unfortunately, any polite requests to allow us to park there usually ends up with a remark like, "I can park where I want" which, while perfectly true, is certainly not very considerate! My parents have lived here for 30 years, the longest out of the residents in this street and in the last 2 years these issues have caused more grief than you can imagine!
Oh well, at least the walks to/from the cars are doing us some good :)