Overfilling the tank. Do a canisterecomy, problem solved.
+1
Just make sure to route the filler drain and vent line somewhere safe. ( I know it seems like a no-brainer, but the last time I got my Triumph back from the shop, they'd lost the original hoses so they'd replaced them with much shorter ones which, the first time I overfilled my tank, dripped gas right onto the exhaust pipe.)
fwiw, since the vent line is actually open to the atmosphere once you remove the cannister, I like to keep it high and put a filter on it, rather than leave it open down by the road. I have mine routed to the side cover, where I zip-tied a small fuel filter to the frame.