Hi,
There was an article in PFK, October I think, about the toxin release.
If I remember correctly it was saying that when stressed they can release a toxin from a gland below the gill. It was discovered after looking into the fact that quite often, when buying Cories from the LSF, you would find a dead one once you got home. The toxins released when in such a small 'bagged' environment meant they poisoned themselves.
It may be a natural reaction to the presence of a predator (stressed). This has gone un-noticed in the aquarium or their natural habitat where the toxins would soon disperse.
It didn't mention anything about stinging though.
Regards
Dave.
Edit: There was a picture in the article which captured the Toxin release.