Pet Peeves, Part 1:
- not writing any log messages
- almost as bad: writing log messages without timestamps
- a server application that picks a different TCP port every time I restart the application, and my 30+ users all had to update their client software configuration with the new port (reboot would start at the same initial port, so I rebooted it a lot)

The last two happened to me years ago when testing network management software from Alcatel called OmniVista. (I had feedback.)