Worked at AOL tech support back in the day and I also still have the occasional flashback to the pain these so called modems caused us all.
Winmodem. Dev hooked up a debugger and found the issue. There was a bug in the soft modem driver. Hot fix was released, but it was too late for the pressed CDs. Luckily it was an edge case on high-end laptops. That were issued to all of our execs with the buggy driver.
Good times.
My greatest win was to add a few lines to our RADIUS server to flip case one time on bad logins, so if 'mYpASSWORD123' failed it would try 'MyPassword123' and let them in if it worked. Logs showed thousands of fixed logins per month and reduced tech support calls to less than a third. We declared victory over CAPSLOCK.