Fall foul of somebody/something
Definition 1 of 2
Pro Tip 1/3Legal Context
Used when someone does something against the law or formal rules. 
If you copy someone else's work, you could fall foul of plagiarism laws.
Pro Tip 2/3Potential Consequences
Implies that there will be negative outcomes, like fines or penalties. 
Companies can fall foul of environmental regulations and face heavy fines.
Pro Tip 3/3Avoidance
Often used as a cautionary advice to not break established norms. 
Make sure to declare all your income to avoid falling foul of the taxation rules.