Tisiana at LS Dru Circle sells you different items than you buy
 

Tisiana sold me a sapphire necklace instead of a bronze collar. I thought i made a mistake so I tried to buy an ale or short beer but ended up with a fire drake scale instead. This might piss some people off.

Thanks for working hard to keep this awesome server going!

-Ghwerig, 44 troll shaman

This is a known bug, it happens when someone else uses a merchant at the same time as you and all the items they have moves up and what you think you're clicking on to buy isn't actually what you'll get.

Does it over-charge or under-charge?

It charges whatever the item is that gets moved up on the vendors list. So if you click on say a 5p item, and all of a sudden someone buys something else from that vendor without you refreshing the vendor window, it'll move up whatever is below that (not sure how it orders this, server side) so if the item moved up costs 200p, you might end up trying to buy that.

At least that's what i think it does. Maybe a dev could clarify this.