It appears that the onebox (and link-detecting, for that matter) mechanism is broken by special symbols (see here).
For example, the URL
https://stackapps.com/questions/2177/stackexchange™-supercollider-freehand-circle™-editor-now-supported-on-every-s#comment10630_2177 is registered in chat (and on sites) as
https://stackapps.com/questions/2177/stackexchange (which still works, but doesn't jump to the specified comment).
If you need any more proof than that, look at what it does here: