I turn off the petcock religiously, even when I'm only stopping for a minute.
My TW was a bumper queen - when I bought it used, it was 3 years old and had less than 500 miles.
Needed a new battery and a carb cleaning / rebuild.
Since I wanted the TK up-jet mod and because I was real busy at work and needed the TW up and running ASAP, I took the carb off and dropped it at the local Yam dealer.
Over the next few weeks the bike filled the airbox with gas four times and filled the entire crankcase with gas twice.
Yes, I am that clueless and distracted.
Never had a bike that needed the petcock turned off before.
The second time the crankcase was filled, leaving me stranded, I had turned off the petcock, but didn't get it perfectly at horizontal.
That time was the charm. Every time I dismount the TW, I check the petcock 2 or 3 times.
A few weeks ago, a house burned down a few miles from here and it was traced back to a gas leak from the bike in the garage.