Woo Hoo !! First try !!
http://i.imgur.com/FPMa3.pngActually it's probably just my fragile system... I will try to reproduce.
Edit: I am able to reproduce... Still, my system is buggy. So that could actually be the problem.
Win 7 that is setup to run from a network, but I'm not pluged in to the network, so if anything ever tries to access My Documents, the whole thing goes haywire...
Anyway, to reproduce: I click on the Pi button, then go to "Own Stored Units." The only thing there is "hogh," which I think might be a built in sample, since I don't remember putting it there. I choose hogh, and get
hogh | 1 hogh
http://i.imgur.com/vbp3X.pngand the hint-toolbar at the top tells me it's a hogshead.
I click <Backspace><Backspace><Backspace><Backspace> to get rid of "hogh" from the left. Then, as soon as I press <Enter> the error happens.
Is anyone else seeing this?
Edit2: Tried to record steps
http://screencast.com/t/lnvGaoMsSEdit 3: Am on my (more stable) Vista machine now. Still getting the bug, but it appears to occur with ANY of the functions, operators, etc. So Opal Calc has 17,456 different functions and each one has a bug. So 17,456 times $50.... Hmmm I'm gonna need Opal Calc for this!
Nah, just messing with you.
Actually it appears to be ONE bug and it doesn't matter what gets typed and deleted. If ever the entry field is empty and you press <Enter>, you get the effect. Or at least that's what I'm seeing. Of could if this does pan out to be a certifiable bug, I have a friend who might like a free copy of Opal Calc

Edit 4: Okay last edit... That hogshead thing wasn't on my other computer which means it's not built in.... Which means I put it there.... WHAT THE HECK IS A HOGSHEAD ANYWAY????