if you have a signature that is just under the 100 char limit, and contains a url using [ url ], I.e mine is:
-- codemastr
[ url ]http://www.unrealircd.com[/ url ]
[ url ][url=irc://irc.axenet.org][url=irc://irc.axenet.org][url=irc://irc.axenet.org]irc://irc.axenet.org[/url][/url][/url][/ url ]

Next time I go to my profile, it reads:
-- codemastr
[ url=http://www.unrealircd.org ]http://www.unrealircd.com[/ url ]
[ url=irc://irc.axenet.org ][url=irc://irc.axenet.org][url=irc://irc.axenet.org][url=irc://irc.axenet.org]irc://irc.axenet.org[/url][/url][/url][/ url ]
Which is now over the limit and therefore I have to manually modify it each time I edit my profile...

(spaces were added so the forum doesn't replace it with HTML tags)