Often when dealing with long commands and having resized the window of konsole or gnome-terminal, the line was not wrapped correctly and I was writing over the previously typed characters. Usually resizing the terminal back to default size helped so I was not really pushed into finding the culprit. But today I got really upset and spent five minutes googling around :-)
And it appears this is a bug/feature of bash itself. In short, when setting the shell prompt, bash can count some non-printing characters into the length of line text. It can be easily cured by adjusting the .bashrc file, as explained in the fine documentation.