Command line input while a running script displays strange symbols

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http://unix.stackexchange.com – If I use the bash shell cli and run some program which doesn't return immediately, then I input some letters (basically typing the next command before noticing the previous didn't finish) and the input always looks something like this (especially when using arrow up and down keys:) ]]A^ or ]]B^ And I wonder, is this kind of premature input 'harmful' in anyway, or what does the returned string mean? (HowTos)