If you try to play Netflix on Safari using an external monitor/projector (either mirrored or extended), you may get this errror:
Whoops, something went wrong...
There appears to be a display problem. Please ensure your monitor is HDCP compliant and is not mirrored using Airplay.
For additional assistance resolving this issue, please visit our Help Center.
Error Code: S7363-1266-48444350
Things may change, but as of the time of this writing, the suggestions Netflix has in its support article do not solve the problem (Disconnect your display doesn't fix the issue; Remove problematic software might work in the unlikely event you have DisplayLink installed; Restart your computer also doesn't fix the issue).
I found another link that had a host of other suggestions: Fix HDCP Compliant Display Error When Watching Netflix Video In Safari On Mac. One of the suggestions (enable the Developer menu in Safari and then switch the user agent to an older version) worked when I tried it in El Capitan but not when I tried it in Yosemite.
The actual solution is just to use another web browser (e.g., Firefox or Chrome).