Piss off the pot.
In most Spanish regions, a 'tiesto' is a vessel of clay used to raise plants. In Castille the word is also a urinal. So 'fuera del tiesto' literally means pissing but missing (the toilet). However the Spanish use the phrase to mean 'miss the point completely' - in other words 'get hold of the wrong end of the stick' or 'bark up the wrong tree'.