My poor light!!!
I have no idea why the battery leaked so badly. None of my other batteries of that type have leaked this way before, and it wasn't old or anything. He battery had produced so much corrosion, that I had to unscrew both the switch tailcap and LED caps off the tube, and press the battery out of the tube! It was so tight, that the plastic label of the battery degloved itself! Yikes! The corrosion ate right through the anodizing, and clean through the aluminum altogether, where it was thinnest, near the threads. It ate into the retaining ring that holds the switch pc board in place.
I carefully scraped out as much corrosion as I could, and neutralized the caustic residue with lemon juice (boy did it fizz up from the acid-base reaction!)
On disassembling the tailcap, I discovered that the grommet that seals the board did it's job, and the PC board had only the most minor traces of corrosion, which were easily cleaned. The corrosive gunk never made it to the LED end of the flashlight, so it too was spared! On putting in a new battery, it fired up right away, and works flawlessly, albeit, with a small hole in the side, that is fortunately covered by the tailcap when screwed on, and resides behind the o-ring that seal the tailcap to the tube.
Still, WOW! It took a beating from that corrosion, but thankfully, it remains fully functional and fully usable! :D
In conclusion: Check yo batteries periodically! Sometimes even relatively new batteries can literally s*** themselves without any warning, and the sooner you catch this type of thing, the less damage that will be done! It's not just a dim light that's good cause to check your battery.