I have until recently had an open bug with Apple from October 2014 that causes the computer to freeze when using the built-in VNC (from the Finder) and various video processes at the same time (whether it's video in Safari, using iMovie, or sometimes just the animation of scrolling a document). It's been present since the first release of Yosemite and is still present to this day. The bug was finally closed as a duplicate of another bug. It wasn't fixed--the bug was just closed.
So basically I can't use the built-in VNC without my computer freezing, and it's been that way more or less since October 2014. No Flash installed. Multiple re-installs. Had the computer sent in for repair and parts were fixed that weren't related to the issue.
I can't count how many sys diagnose files I've sent to developer support. It's been over a year, and they acknowledge it's a real problem but just don't ever fix it.
I've been using OS X since the Public Beta in 2000, and I can say that its bugginess has increased in negative correlation with its price. Back when you paid $129 for it, it worked well. Even the public beta cost $29.95, and while it was incredibly, incredibly slow, it never froze.
I have to restart for freezes now 2-3x per day.
EDIT: To be clear, this bug started in Yosemite (10.10.0) and is present still in 10.11.1. I'm not taking any chances on 10.11.2 yet. I was so hopeful after so many reports I sent in that this would have been fixed in El Capitan. At this point, I don't think OS X will ever get the attention it needs.