Some time ago I ran an experiment which measured (as a placebo thing) how often loading a script fails. The details are in the CORS blocking metrics wikitech-l thread, but it was in the 0.1% range. This is a lower estimate - the measurement itself was done via JS so certain kinds of JS loading failures might have prevented the logging from happening at all. Nevertheless, it is maybe of interest - for me it was a surprisingly high ratio.
