Originally posted by Mr. X:

Originally posted by Periost:

..
.... and I´m 100% sure that no one of my friends using mods or console comands....

Why are you sure? Because they told you so?
Would someone who summones bosses with devcommands tell you the truth?

Deine Freunde verarschen dich!

I notice both times OP was playing with friends when the bosses happened. And while on one hand I would note that one friends name from the previous seed, I would also suggest OP test said friend variable by OP starting a map on their own, and playing it alone for awhile to see if the game *does* decide to spawn bosses on them. This is what is called a control group of an experiment.

Should the control server spawn no monsters, then you will know that the game itself is guaranteed to not be the problem and that you now have a list of friends to watch.

My next step would then be to play with your friends, and while playing, keep a close eye on the who is on list by pressing F4 or F2. This will bring up server stats and whoever is on at the time, including players who dont want to keep their name visible on the map. And when a boss suddenly appears to ruin your day...

You now have a list of people at hand who could have done the deed. And anyone trying to be sneaky by keeping their name invisible from the map will have all but painted a big red X on their chest.

Because if its vanilla, the game wont spawn bosses randomly. Someone is having a giggle at your expense. Or they might think its a funny prank to play.