I have my own story where I've done something I've regretted, but one from a friend first.
Back in 2011, we needed an inventory system. Because it needed to integrate with various existing systems (some of which were custom designed), myself and a couple of colleagues built it from scratch, using SQL server, Java, PHP and other web technologies. We had V1 up and running, and I was adding equipment to it, using the site we'd set up for this purpose. All of a sudden, I started getting errors saying it couldn't find the database. I checked the server was up. It was. So, I logged on to SQL Server Management Studio, and couldn't find the database. I got our DBA to look at it, as he has access to the backups, which I don't. As far as we could determine, one of my colleagues had renamed the database to a full stop, which was apparently preventing the GUI showing it. The DBA got it back, and immediately locked down the database so no one apart from him and another colleague of mine (not the one who renamed the database) could make structural changes.
My one was when I first started. We were using Windows NT 4, and Microsoft had just released SP 2. I had been testing it for weeks on my machine, and, in my defence, it had passed all the tests with flying colours. So, I confidently started installing it on staff machines. Roughly half of them failed, and had to be re-installed.