In the past there were some APC systems that failed in a epic fashion where the system would over charge the batteries or a cell / cells would overheat and begin to expand.
It's arguable that it was by design as APC was also in the business of selling very over priced battery packs.
The good old days of IT being as many free UPS as I could carry, only that they all needed new batteries and some needed full disassembly to remove them!
I personally never saw one on fire but did attend a site where the cooms room was VERY warm and VERY smelly with a somewhat rotund APC bleeping away.