I spent about 2 hours investigating an interesting problem with IETester - frankly I don't know whether it's a bug or by design but it doesn't seem right to me.
I'm using IE 9 on Win7 and IETester v0.4.11. The site I'm testing (which I can't share) is on IE9's compatibility view list published by Microsoft. So when IE9 is set to use that list and I test the site in IE8 mode IETester switches to IE7 Strict rendering (and I'm seeing bugs which I don't see under a native IE 8 installation as the site is not on the latest IE 8 list). If IE9 is set not to use the MS list I get proper IE8 Standards rendering.
So it seems that IETester takes into account the IE9 list into account for displaying IE6/7/8 rendering that doesn't seem right to me because:
a) I don't see why it should read the list anyway or at least this should be controllable behaviour
b) The IE9 list is about IE9 compatibility - IE8 has its own list. Besides the IE9 one is somewhat different (flags etc)
c) Confusion ensues...