From the Hohlspiegel column of Der Spiegel, which is intended for funny/interesting sentences from other sources:
Schild an einem Fischweiher in Saarbrücken-Ensheim:
Baden, Verunreinigen, Einlassen von Hunden, Angeln von Unbefugten ist verboten!
I guess the reason this is funny is the notion of unauthorized people being fished (instead of them doing the fishing.) In that sense, "Angeln von Unbefugten" is ambiguous. How can we make it clear? Would it be "Angeln für Unbefugten"?