Make a mountain out of a molehill
Definition 1 of 1
Pro Tip 1/3Idiom Origin
Understanding it evolved from the literal molehills helps grasp the exaggeration aspect. 
It's just a small error, no need to make a mountain out of a molehill.
Pro Tip 2/3Figurative Use
It's not used for actual hills or mountains. It’s strictly a metaphor for problems. 
I only mentioned the issue once, don't make a mountain out of a molehill.
Pro Tip 3/3Informal Tone
The phrase is common in casual conversation, but might not suit formal writing. 
You're making a mountain out of a molehill over this, let's just relax.