Doing some innocent testing of MDL-38743 it was discovered that etag verification (and last-modified verification consequently) were no matching ever.

See point B2 in that issue.

This is about to fix the etag matching over all stable branches.

It has a big impact in pages heavily using stored files (forum discussions… long books… long courses with labels…)

I reviewed all the rest of etag uses in code base (js, css, yui combo…) and all them seemed to be using the /revision/ approach, so they are not affected. In any case I'd recommend to review all them again, just in case I missed anything.