There are a few subreddits that are pretty much in direct opposition to what their name suggests.

It's akin to domain-name grabbing and land runs, where the first person to claim an area gets to control it forever.

Places that this has happened include:

• /r/feminism/ — overrun by MRA folks [1] [2] [3]
• /r/republican/ — overrun by liberals
• /r/religion/ — overrun by atheists
• /r/stormfront/ — all about weather, nothing about race (it sometimes slips into parody)
• /r/lesbians/ — mostly guys hang out here; women had to move to /r/actuallesbians/