${
However, I tried the literal
'${'
Which also breaks it. In fact, it looks like anything that has ${ in it will break it, anywhere at all in the search string.
Also, if you close the parentheses, e.g. ${}, it fixes it. This works with any number of leading ${, e.g. ${${${${${}
not sure if that was intended or not.
http://i.imgur.com/OrqtK.png
Loading http://www.google.com/search?q=${ does what you see, but entering ${ in to the search box and pressing enter does what everyone else sees.
edit: compare your clean human generated address bar to the other screenshot's messy software generated one.