The most recent major one I know was the LTS release, in which they seriously broke embedded content like Flash and Java applets.

The issue was known and in the bug tracker several days before the release, yet they went ahead and released anyway. Consequently many sites and browser-hosted user interfaces across the world were broken.

It was then a very long time before another patch was pushed to fix the regression.

I'll leave it to you to decide whether it's worse that such a serious bug could get into the repo in the first place, that they still pushed the update to everyone even though they knew about the problem, or that they allowed sites that rely on these technologies to be broken for so long before correcting their mistake.