The stylesheet of this file should not validate, due to a missing octothorpe in color definition.

I show you the styles I declared here:
div#main {
font: 100%/1.5 Verdana, Arial, sans-serif;
color: #8D2828;
padding: 1% 2%;
border: 1px solid c85;
}

Now, I guess you, like me, would expect an error message from the validator. Surprise, this is what I got:
(I chose CSS2 Profile, All Warnings, All media)
Valid CSS. Screenshot of Validator Output

Okay, let's push it a bit further. I replaced the last declaration with:
border: 1px solid 123456;
No effect, it validated as well.

Okay then, next try. Changing last declaration to:
border: 1px solid 8D2828;
Ahhhhh.....didn't validate, but where is the difference?
Not Valid CSS. Screenshot of validator output

Now, we're making it even more weird: Changing the CSS profile form CSS2 to CSS3. I took the first declaration from above: Yak! Validates.
But now, !important is added to my border declaration.
Valid CSS. Screenshot of validator output

I validated the CSS using the following address:
http://jigsaw.w3.org/css-validator/

Go figure! Validate this document.