There are two very different phenomena that are both called flash mobs, this has been the case for like 10 years now and both phrases are in common use but it seems people who use it one way rarely encounter people who use it the other way and there's a shit ton of confusion over what the term even means yet I haven't ever seen anyone address the confusion.

A lot of people use the term flash mob to mean a seemingly spontaneous meeting of people in public, arranged beforehand on the internet, that usually involves zaniness like people dancing or doing something else goofy like wearing pajamas on the subway. These people are often typical nerdy white folk, college aged and above who use tech a lot in their lives.

A lot of other people also use the term flash mob to mean a seemingly sudden and large group of young people committing petty crimes together and openly with their general attitude being 'they can't get us all', usually they'll ransack a convenience store or gas station and they'll all walk out with beer and snacks without bothering to pay. These people are often black americans who are usually under 25 and as young as 12. There is a secondary issue with this usage of the term here because there has been a bit of a media panic around this form of flash mob, very very similar to the 'knockout game' phenomena and related panic, middle white-america is terrified of these flash mobs and they have attracted a lot of attention from commenters online and there is strong racial undertone to this issue.

Both terms have been in common usages for a long time now (10 years it seems) and both types of incidents have been recording happening a ton of times in the past 10 years across the country. Somehow though it seems the nerdy middle class white people who like harry potter and internet memes who use the term flash mob to mean one thing haven't much encountered the lower class white people who debate ford versus chevy and listen to conservative AM radio and take it to mean another. Just like you know a flash mob to be dancing (as I first did) and would scoff at any other use of the term a lot of people take it to mean shopplifting en masse (or worse) and wouldn't be familiar with the other meaning.