Press apple + tab and cycle away onto another program. Then access apps-utilities-Activity Monitor, open it up and force quit minecraft process.
That message doesn't mean anything but that it's shutting down the game. As I said, if you want to know why, you better provide some logs. And I recommend when you're getting in the second time, do not go to full screen. If you need to modify it first before you start the game, go into minecraft folder, find file that says options.txt, open it and change fullscreen from true to false and save file. Then start the launcher.
all this incompatible mods talk is absolute bs... because im getting the same issue but was able to use all the mods i have atm perfectly fine for quite sometime... one of the major issues i have is that galactic craft mod and biomes o plenty i believe stopped working together because they made the new versions very badly.... and couldnt get an proper answer in how to fix it... thnx to how bad galactic craft became i had to stop playing minecraft and havent played since, would like to but its a huge fking headache to fix....
also from time to time i can run a single player map for more then a few seconds and then the server shut down bs happens... which also means the mods are fine together...
.... the lack of updates from the owners of minecraft is pathetic and imo move on minecraft is a dead game due to how many issues it has...such a shame too
There were no logs (and by logs I mean crash reports, launcher log+game output from mc launcher or anything else relevant to the issue that's requested) since september last year which I'm not going to bother answering as the user is long gone. Now...if you have personally a particular problem, submit logs or you're not getting any help. We're not psychics and can't magically know what's wrong from two sentence explanation.
Time: 11/02/15 12:48 AM
Description: Unexpected error
java.lang.NullPointerException: Unexpected error
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
A detailed walkthrough of the error, its code path and all known details is as follows:
-- Head --
You have a mob spawning mod that's derping. The error doesn't name which one so it's going to take some work.
First back up your world before you do anything cause taking mods out will corrupt it. Or make a fresh one.
Then, start taking out mob spawning mods one by one. Start with obvious ones like butterflymania, Twillight Forest, Oceancraft, rancraft penguins and then continue with any mods you know that spawn mobs. You should be able to nail it pretty quick.
After you know which one, contact the author of the mod, explain how you got the error and that only addition of that mod triggers the crash and they should be able to help you or you might just have to drop the mod in question.