So, I forgot to eat, and got sent to hell.
I spent a few moments squishing grapes like crazy and got resurrected, and on my return, noticed a bunch of messages rapidly fading away - those growl-like popups that appear down the left hand side.
They all said the same thing, something along the lines of:
A wee problem, couldn't find the itemstack(0)
That may not be a verbatim quote - the messages quickly disappeared.