When I tried to change from super user to common user and therefore typed "exit", I got an error message, that a job was stopped and I could not log out as root. So I listed the jobs:
#jobs + yes | apt-get install build-essential
I moved the job from the shells background to foreground
now I thought, I could simply press ctrl+c to end it, but each time I pressed ctrl+c, nothing happened except for "^c" being displayed. When I pressed ctrl+z I came back to the prompt, but the job was still there, so I was caught in the # .