NeoGAF user Trike claims, after messing around with the Wii U's Miiverse, that his console suddenly allowed him to access the service's debug/admin options.
As you can see in the pictures posted above and below, he appears to have access to screens that let him monitor users, reset their passwords and even delete... something (whether that's delete a password or their entire account is unclear).
Now, you might be thinking that seems a highly unlikely thing to happen. Which it is. But then, this is Nintendo's first real try at this home console internet business. There's going to be missteps and mistakes. This might just be one of them.
I wonder what that Yoshi's Wii U thing is. Most commonly-used words on the service right now? Or, as is being speculated, something to do with Miiverse demos?
UPDATE - OK, fun's over, Nintendo closed the loophole. For the record, here's how Trike claims he accessed it:
Go to your Miiverse menu. Hover of the "X Close" button, and literally press X on the gamepad. Boom, you just became a master hacker. At least you would have been like 20 minutes ago when it was still working.