Not exactly a false friend, but an illustration of what can happen if you push a grammatical rule too far.
I was living in Valls (near Barcelona) and wanted to buy something to cheer up a friend with flu. The local sweetie shop had lots of chocolate animals -- this was just before Easter. "Just the thing," I thought. The one I chose, not dreaming of the linguistic minefield lying in wait, was a chocolate hen with some chicks. Now, I knew there was a rule that masculine words end in -o and the corresponding female words end in -a. OK, pollo is chicken, so hen must be polla. In I went and said to the assistant, "Could I have that polla please?" and pointed to the animal in question. Not a flicker of a smile crossed her face as she replied with a word I didn't know, "gallena" (not sure of the spelling now -- this was 15 years ago). Fearing some misunderstanding I repeated "polla". "Gallena" came the reply. "OK, have it your way," I thought and repeated "gallena". Clutching the chocolate I went and handed it over to my friend and asked her what was going on. She nearly fell out of bed laughing. Apparently "polla" means the same as cock, but only in the non-poultry sense of the word.
Bingley
