I also had austriaco's problem with long urls ending in "-", which was easily fixed. But I've another....
This link:
http://combatwombat.7doves.com/?tag=thunderbird-email-signatures-background&posts=50
Ranked 3rd in Google, doesn't work. It's the same one that alerted me to the other issue. But as the above URL is not the Permalink, I'm lost as to how to fix this one.
Permalink: http://combatwombat.7doves.com/2008/06/16/thunderbird-email-signatures-background-
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Okay, I don't know how or why, but the tag had been removed.
I edited the initial post, putting : thunderbird-email-signatures-background into the tag line.
Now the url resolves.
Sorry for the noise. I'm the bug.