Streamlight TLR-6. Known design issue though don't tell anyone important at Streamlight that someone who works there told me that. Screw bosses are not reinforced sufficiently (float in the case, no webs attach to side or case. Can shear away when over-tightened (as part of install) or weapons is dropped, etc. Unclear how damage occurred, noticed loose light at pre-service inspection. One did for me, light continued to operate but was somewhat loose on the pistol. Laser would have been deeply not-zeroed (I do not use the laser but the light moved a lot). Manufacturer mailed new case (for free, in 2 days), which was user replaced.
Surefire X300U-A with DG switch, mounted to M&P 9mm 4.25. Carried in Safariland 6360RDS. Exposed to running around in all weather conditions (weird day: rain, freezing rain, snow, sun). Mud and banging around, etc. Not immersed, dropped, wrestled with, etc. Gun was fired appx 40 rounds over the course of the day. Some hours later cleaning up, hosed and brushed off items including pistol and holster before detail cleaning due to silty mud. When taking guns apart some hours later, the X300 was on at a low power; hit any button, goes to full power but when released goes back to the dim on. Removed light from gun, removed batteries to turn off. No obvious water ingress or damage to light or switch. Air dried, cleaned with alcohol, etc. Worked after that. X300U-A on carbine carried the whole time had no failures.