Ability boost to Con

[Martial_law]

If a boost changes your Con modifier, It does not apply retroactively for previous levels, right?

[Snowbluff]

You mean for HP? Yes, it applies retroactively.

[Thrice Dead Cat]

This can lead to some pretty nasty side effects if you happen to be a barbarian whose rage wore off in the middle of a fight (thus possibly putting you past -10) or if you have a character take CON damage from some poison, venom, or other effect.