There are tons of these voters, although it's not common that they get their own precincts. Vernon is probably the most extreme example. Industry, California, also has a considerable number of these voters.
In theory, all of these registrations are universally illegal, either because it's not a valid residence or because it shouldn't be a residential address. In practice, election workers lack the resources to check to see if a claimed residence is legit or not. Back after the 2004 election, the Washington GOP challenged a bunch of residents they thought were improperly registered. They ran into some problems with live-in managers at hotels or mailbox stores, which are legal, as well as artists subletting lofts, which are technically illegal, but not really the Election Department's business. Another example of "false negatives" are homeless, who tend to be registered either at the county courthouse/elections department, or wherever they get mail. And then a lot of their challenges were just screw-ups, where there may have been a mailbox store at 123 Main Street East, and an apartment complex at 123 Main Street West. Even after that, there's the big question of whether people should be prosecuted -- it's a felony, but it's a felony that's easy to commit unthinkingly.
It's pretty rampant, though, and essentially impossible to fix. But there are some particularly blatant ones.
This address, for instance, is a Mail Box West store with no attached apartments. Nobody should be registered there. But for a registration challenge in most states, Washington included, you have to provide evidence of where the voter
actually lives, not just that he doesn't live where he claims to.
