I know of one glider here in Canada which burnt up while in the trailer as a result of a metal rigging aid falling onto the battery (placed in the trailer's cargo area) and shorting it out. I also had a few pictures, lost when an old hard drive died that a club member sent me of a glider that had the battery short out and burn fortunately before takeoff. Recently someone I knew improperly charged the batteries that reside under the cockpit floor of his ship and I think he's lucky that the cases didn't burst - they were swollen so much that getting them out was rather difficult. All were SLA batteries.

These incidents say more about the need to protect the battery terminal area well and include a fuse as close as possible to the terminals (a set up like the Dittel battery box for example) and use a good charger than they do about chemistry though.