Why is return is ignored during output in bash?

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http://unix.stackexchange.com – When I start a program that runs for a bit, say make, I could just type the next command in bash even before I got a prompt back and when the first command finished, the second one was executed as if I had typed it later. But suddenly this doesn't work any more. The additional command will be shown at the prompt after the first one finished but is not executed. If I type more commands, they are all shown at the prompt (without any space or newline between them). Any idea what is at fault here or how I can figure it out myself? (HowTos)