Here's one to start you off..
In English we have a saying 'Splitting hairs' when arguing over a trivial difference.
In Spanish they say 'Hilando fino' which translated means spinning fine (as in spinning a thread)  - hilar is the verb 'to spin'

Also, be careful in Spanish as pronunciation can be critical...
Happy new year is 'Feliz año' or 'Feliz año nuevo' (the Spanish tend no to say the nuevo as it is assumed) however not pronouncing the ñ (enya) makes it 'Feliz anno' or 'Feliz anno nuevo' which translates to 'Happy asshole'.. so be warned!