Well, unfortunetaly, there are MANY trick like:

submit="document.layers
[layerName].document.formEmail.submit()";

And these cases are really hard to handle, asd 
therefore there might be some problems mirroring such 
sites. 

Javascript is generally a problem, as the engine would 
have to be "smart" and "understand" the way the code 
works to get pages.

Some sites, with pages like:
foobar.src="foobar.gif";
CAN be parsed and work well with HTTrack.

But cases like:
foobar.src=a + "-alt" + aext + ".gif";
CAN NOT be handled, as complex javascript code is 
really impossible to hande

The thing-is.com uses many javascript code, very often 
COMPLEX code, with many frames and cross-reload pages, 
and unfortunately can not be mirrored successfully