"Disallowed Key Characters" for HTML content - even using $_POST?!

[eluser]mattpointblank[/eluser]
I feel like this thread might be related: http://ellislab.com/forums/viewthread/65152/

That user was having issues if his textarea contained the word 'method'. The function that's giving me this error message is supposed to be checking keys, not values - eg, shouldn't it be validating my form field NAMES, not their content? My form field is just called 'html' - the error I get is a bit of weirdly converted HTML:

Code:

Disallowed Key Characters: nbsp;</p>
<p_style

Anyone got any ideas?