I often here this from veteran players: everything is ok as longas there are No Ninjas in the refrigerator.
The idea is simple: you go to make a sandwhich in the middle of the night you open the fridge and are attacked by ninjas for no particular reason. The event makes no sense to the context or the game at the time. It throws players off and derails game plot-lines. So I've been told.
However, I can't count the number of times I've read a RPG which states in case of boredom or a scene slowing down "have some minions kick down a door with guns blazing."
I admit I've used the minions w guns options a few times sometimes to great success and other times to horrible story telling failure. That said when a player says, "No Ninjas ..."
" ... are they saying: no uneccesary risk, please?"
There are times when "random" events will occur and seem to have no relevance. A thief drops a gem from a bag which rolls down the street and lands at the feet of the bard of the party. The bard picks it up and a storyline can be built from this.
or
A band of heroes is attack by a goblins during a storm and getting thrashed badly. The damn breaks the floods waters rise and the goblins are washed away but the party survives do to teamwork and quick thinking.
Are these unreasonable events? Are these ninjas? If we say "No Ninjas ..." does this rob the GM from spontaneity and a key tool in his toolbox?