Placing poison in a store causes it to no longer stack properly with others of its own type. Instead, it will only stack with poison that has likewise been in a store.
More importantly, poison that has been in a store appears to stop working entirely. I have done some testing with another player using two different vials of poison: one that had never been in a store and one (formerly of the same stack) that had. The poison that had been in a store completely failed to do anything, whereas the "fresh" poison worked fine.
The above behavior has been noted with both Eyeblast and Carrion Crawler Brain Juice. Haven't tested it with other poisons yet, but would not be surprised if there's a similar problem.
