This has worried me for quite awhile, and i finally have solid proof that something is weird. See my consecutive posts in this thread.
In the first, my sig looks exactly like how I'e defined it in my control panel. The second one has several extra newlines for no reason that I can comprehend.
I haven't noticed this in anyone else's sig, so its always bothered me a little.

P.s In case its a browser problem (I use konqueror), this is what I see often: http://img98.imageshack.us/my.php?image=siguh9.jpg
P.P.S yes I'm drunk but this is still a valid query IMHO.
Brief search did not seem to show anyone else having this problem.

I don't have signatures enabled but looking at the posts you pointed out without signatures seems to have extra empty lines below post #3 that you pointed out as well.

All posts have a minimum height due to the amount of profile information in the left box. You're just seeing the empty space that results from posts not long enough to fill in that space.

--jeremy

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One of the funnier things I've read here recently.

I didn't even think about the left profile column and I'm sober right now.

Brilliant! I never really noticed that box, it makes perfect sense now.