You have a label with a nested checkbox input element like so:

<label><input type="checkbox" value="foo">foo</label>

When you click in the label area, all browsers except IE6 will fire the onclick event for the label followed by the onclick event for the input. IE6 will only fire the input's onclick if the label's for attribute is set and points to the id of the input.

<label for="x"><input id="x" type="checkbox" value="foo">foo</label>

Also, both IE6 and IE7 will incorrectly render the following style:

label { background: #def; padding: 2em 4em; }
input { margin-right: 2em; }

Firefox

IE

And the icing: neither IE6 nor IE7 will fire the label's onclick event for clicks occurring in the marginal space.