When you select text in an input field using the keyboard (using C-Space then C-b/f/p/n for instance),
C-g does not cancel text selection mode. The only way I know of to cancel text selection in this case is
to copy or cut the selected text using M-w or C-w or to press C-Space yet again. Besides this differing
from the behavior of Emacs, that behavior agrees with the overall meaning of C-g.

Exact steps to reproduce:

(1) Select a text input field on a webpage and type something.
(2) Press C-Space.
(3) Press C-b one or more times to select the last character you typed. 
(4) Press C-g.
(5) Now observe that whatever cursor movement commands you execute (C-f, C-b, etc.), you are still
selecting text---even if you leave the input field and come back to it.

The ultimate solution to this is probably to change the binding of stop-loading so that C-g can behave
more like it does in emacs.  As long as C-g is used for stop-loading though, I'm not sure that we would
want to have the key do something different inside of text boxes than it does outside of them.
Improvements to Conkeror's focus system would also bear on this, because currently, a website is able to
automatically focus a particular widget (often done for search boxes).  In such a case, if C-g did not
call stop-loading, the user would not have a way to stop the page load.