A tempest in a teapot
Definition 1 of 1
Pro Tip 1/3Overreaction Warning
Use it to suggest someone might be overreacting to a minor issue. 
You spilled a bit of water—don't make a tempest in a teapot.
Pro Tip 2/3Conflict Context
Often used in the context of disagreements that are blown out of proportion. 
Their heated debate over such a small error was just a tempest in a teapot.
Pro Tip 3/3Idiomatic Caution
Be cautious—idioms like this may confuse non-native speakers who take words literally. 
To non-native ears, 'a tempest in a teapot' could sound mystifying.