Strange float behaviour and rogue LT symbol

I'm putting together http://konrad-grabowski.com and the artworks on the front page should just float one after another, however after the eight floating element, things start going wrong. I'm not sure why; could someone take开发者_开发百科 a look?

Also, upon refreshing the page, sometimes a rogue less-than symbol appears to the right of my content. My guess is that this is related to the float problem, but I can't isolate the symbol in my mark up.

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Look at the last three errors when you validate the page...

W3C Validator

You have open elements which should explain the random < or > symbols you're seeing.

You also have a comma at the end of this line which should be removed. IE will probably choke on it.

speed: 700,

EDIT:

In my Safari web inspector, it appears that your "back to top" paragraph element is much wider than it needs to be and it's over-flowing into your gallery area pushing those images over and out of the way towards the right.

Temporarily remove your "back to top" element to see if it's the culprit and then try setting the width here instead...

#back-top {
    width:  108px;
    position: fixed;
    bottom: 30px;
    margin-left: -150px;
}