Debian 11 "bullseye" released

If you really had stable (instead of buster) in your sources.list (new installs usually get buster not stable), then when Debian released bullseye, apt/apt-get update gives an error due to stable changing from buster to bullseye, and that might not be what you expected. With apt you get an interactive prompt that lets you ack these changes, with apt-get you just get an error and can't update until you pass the --allow-releaseinfo-change option.