My favourite piece of DM dickery that is a perfect yin to railgun's yang is the Immovable Rod story.
In short, out of sheer malice a DM decided that the rod is indeed immovable, but not in relation to the ground, but to some point in the universe, so upon activation he made it launch at several thousand miles per hour in some direction.
The players have, of course, turned it on him, by using it as a siege weapon on the basis that an iron rod slamming at that speed is bound to cause some damage.
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