When training, don't use stacks of invis pots. If you successfully steal just part of the stack it converts the ones you stole into night sight potions. The graphic stays the same, but the description changes and so does the function. Quaffing what you think is an invis pot and finding you're still visible, and the lights just turned on isn't terribly helpful. Doesn't happen if you fail, doesn't happen if you steal the entire stack, and when you do split the stack, the ones left behind are unaffected. Bug repeated, verified and report submitted btw.