Almost certainly the selection bias plays a major role - surgeries done early in a week are different than those scheduled later in the week.
Yeah, this reminds me a lot of the "Impossibly Hungry Judges" problem[0]. [0] http://nautil.us/blog/impossibly-hungry-judges
...this has potential!
Scrape GitHub, find closed issues marked 'bug', do a blame on the offending line or lines, find the timestamp and plot.