Whether it be in the URL field or in the notes of a task, %20's that usually represent a space in a url become actual spaces, breaking up the link.

So, for example a link such as....

http://www.website.com/test%20test.html

would become simply

http://www.website.com/test test.html

the link being a link and the 'test.html' being just regular text after the url.