Tell users if they try to publish something with a reserved word

[zwrawr]

E.g. in this case where a user made a game called karma https://ldjam.com/events/ludum-dare/40/$61146/karma-finished

[mikekasprzak]

Also, use a different behavior for posts/games. Allow the reserved word, but emit a slug that isn't the reserved word (i.e. karma1).