This was long ago and the details are hazy. Back in 2003 or so, I was in tech support for a company that used desktop computers running java applets to connect

This was long ago and the details are hazy. Back in 2003 or so, I was in tech support for a company that used desktop computers running java applets to connect to a mainframe via Telnet (IBM Host-on-Demand IIRC). Most of the core business processes were handled by mainframe apps, which the company largely developed. I used to hang out in the data center with the mainframe guys who coded in COBOL all day. On a Friday afternoon, I was working on testing deployment of an update to the java terminal client applet. Everything seemed to work fine in testing, and it was a minor update, so (idiot me) I went ahead and pushed it to the server. Shortly after I pushed it out, the mainframe guys' phones started ringing with complaints that the mainframe was down. Then my phone started ringing. Then all of the phones started ringing. Turns out, something I did in the update (I honestly can't remember the specifics now) reset every local users' mainframe connection information for the applet. Across the whole company. So as soon as they exited the applet, they couldn't get back in. That was a fun weekend.