serious error has occurred while loading moonlight 1.sims 3

Oh. That's happened to me but only when it was loading. And I don't even have Supernatural. All I did was try again but are you getting it when you're loading or when you're actually trying to start the game?

When I am loading a saved game

Here's some tips on the issue: http://simswiki.info/wiki.php?title=Game_Help:A_Serious_Error_Has_Occurred

so now it's 2017 and i'm having this problem.  apparently fixing it isn't a priority to EA.  ☹️

Ea is no longer updating the Sims 3 game. It's considered finished, so there will most likely be no other error fixes unless they feel particularly generous. What solved this error for me was restarting my computer and making sure all folders that opened to the Sims 3 folder were left closed.

Good luck!