The CSS for serving to IE8 & 9 is split up into smaller CSS files to avoid the 4095 selector limits these browsers have.

The code has some kind of check to see if it's splitting in the middle of something it shouldn't but it doesn't seem to be working.

If I visit qa.moodle.net in IE8 the theme is totally broken. If I view source and find the CSS url it'll be of this format:

http://qa.moodle.net/theme/styles.php/_s/clean/1396022733/all/chunk0

and it looks something like this:

@import url(/theme/styles.php/_s/clean/1396022733/all/chunk1);
@import url(/theme/styles.php/_s/clean/1396022733/all/chunk2);
}.restore-course-search .rcs-results

That closing } should have been added to the end of chunk2 file, not the start of chunk0.

Similarly at the end of chunk1 it just tails off with:

.filemanager.fm-loading .filemanager-toolbar,.filemanager.fm-loading .fp-pathbar,
.filemanager.fm-loading .filemanager-container

and the chunk 2 starts

,.filemanager.fm-loaded .filemanager-loading,
.filemanager.fm-maxfiles .fp-btn-add,
.filemanager.fm-maxfiles .dndupload-message,
.filemanager.fm-noitems .fp-btn-download,
.filemanager .fm-empty-container,
.filemanager.fm-noitems .filemanager-container .fp-content

Then chunk2 ends with:

.restore-course-search .rcs-results

we saw at the start.

Possibly this is only happening on master? I don't seem to see any problems with moodle.org, though the relevant code hasn't been touched in a while so perhaps it's just random whether your CSS will be affected by this or not, depending on exactly where the cut falls.