Calling foot faults is tough at the amateur level. First of all, it’s difficult to see completely clearly that an opponent is certainly foot-faulting. In doubles, it’s slightly easier to watch for, but still difficult to call.

Other than foot-faulting, the number one rule problem in amateur/recreational tennis is line-calling. Of course there will always be bad line calls on purpose and otherwise, but there’s an even more fundamental problem among tennis players (mostly at the 3.0-3.5 level). They just don’t get that if the ball touches the line, it’s good.