When left to themselves sims will occasionally do mischief pretty much as in real life. When you play in a tiny lot friendly interactions counts much more so it should be easy to counter the occasional mischief. The only way to stop it would be to disable autonomy for played sims though generally the more friendly they are with each other the less likely it is that they will do mischief autonomously. Notice that if you direct to do an act of mischief it will be much more likely that they will do it autonomously.
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