fall foul of sb/sth

fall (a)foul of someone or something

 and run (a)foul of someone or something
to get into a situation where one is opposed to someone or something; to get into trouble with someone or something. Dan fell afoul of the law at an early age. I hope that you will avoid falling afoul of the district manager. She can be a formidable enemy. I hope I don't run afoul of your sister. She doesn't like me.
See also: fall, foul, of

fall foul of

come into conflict with and be undermined by.
2004 Sunday Business Post Australia's biggest wine-maker, Foster's Group, is the latest company to fall foul of the wine surplus, which is set to continue for at least two years.
See also: fall, foul, of

fall foul of ˈsb/ˈsth

do something which gets you into trouble with somebody/something: They fell foul of the law by not paying their taxes.Try not to fall foul of Mr. Jones. He can be very unpleasant.
See also: fall, foul, of, Sb, sth