I was blowing fuses on my bike after running for 10 minutes so I started to search for a loose connection or worn out wire. I removed the headlight, checked all the wiring then removed the back fender and checked every connection, no shorts or wiring issues. Put in a new fuse which would last around ten minutes then pop. Before the fuse popped I noticed the left hand rear light was slightly brighter than the right so I removed and replaced the bulb . Viola! Problem solved. I never had a bulb cause a short like that before. It will be the first thing I check next time instead of the last. I changed to 2 way bulbs on the back signals in case you're wondering.
Over the years, I have seen lots of bulbs do this. Usually the filament from one of the two circuits will melt and fall across the other one, causing an internal short to ground in the bulb.
Never would of guessed. Good to know information.