In the Pewter Museum of Science, DON'T TALK to any of the attendants when not between the counters. Responding YES if you want to exit the museum might make the game crash instead, and you'll have to continue from the last saved point.
The PCs in the Pewter and Cerulean Pokémon Centers work. Don't deposit any Pokémon in there. They will definetly get ereased in future versions of the game.
You can encounter roaming Pokémon after the events in Pewter City. These are at very high levels, so don't try to catch them.
The Town Map (both in Poké Centers and the item) don't work. It opens a sample region instead.
There are some black areas outside maps:
On Route 1, left and right sides
The guide in Viridian sometimes faces away from the player.
The sign in Viridian skips two lines instead of one.
You can walk though the table in Green's house and through the right crate in the Lab's back room.
The fat man on Cerulean Cape sometimes blocks the shortcut exit.
The trainers on Nugget Bridge continue to get an exclamation mark after you've battled them - although they just give their "after battle" speech and don't challenge the player again.