AHHH NOOooo!
Happened to me once, then I figured all kinds of mechanisms to prevent it from happening again.
1) When considering placing another city I would check how many more border expansions I planned to get in my core towns, and also the status of the border-culture-wars: when you mouse over the tile and it says 55% Russian, 45% Malinese. I'd monitor it for a few turns and decide how quickly I was overtaking the computer and make a guess at how many tiles I would get like that.
2) I don't think it's enabled by default (?) but the HoF mod has an option to report your domination status (including tilecount and target) as soon as you start getting close to it.
Combining 1+2 I arrived at 3, aka "desperate times call for desperate measures"...
3) If your nice game is going to be foiled by accidental domination, gift cities to the AI

that's a surefire but last-ditch effort to reduce your land area!