2016 Leap Second Bugs
The end of 2016/begging of 2017 had a leap second added to the time. A leap second can cause computer software to crash, as described in this article. Luckily this year, the leap second did not do much damage.
The most publicized issue was a DNS error in CloudFlare's infrastructure. The software cloudFlare was using assumed that time would never move backwards. But during the leap second, time did move backwards for a second, causing a DNS bug. Luckily CloudFlare engineers were able to identify the problem and find the bug in about 90 minutes. The fix was actually just changing a single character in the code.
As they put it: "The root cause of the bug that affected our DNS service was the belief that time cannot go backwards. In our case, some code assumed that the difference between two times would always be, at worst, zero."
You can read more about the issue they had and their solution on CloudFlare's blog.
Beyond this, there was no large issues reported. I heard reports of some old systems running into bugs and needing a reboot, but that's about the extent of the 2016 leap second bugs. Mostly old routers and a few old versions of Linux that have not been properly patched. Nothing critical.
Overall that it was a pretty great leap second. I expect the next time a leap second happens it will be even less painful.