Why is my keyboard typing the wrong letters?

While surfing with Iceweasel, I tried typing in a Web address, but I found, that every few keys, it started typing other letters instead of the ones I typed. For example, if I typed "c-c-c-c-c-c-c-c-c-c-c-c" it printed "ckexccccckex" or if I typed "a-b-c-d-e-f-g-h-j-i", it printed "abckexghi" instead. - Each time this occurs, the misplaced letters are different, but always in some pattern. Sometimes, it places backspaces or spaces, not just letters. - When I did not type anything, no words appeared. - When I unplugged and replugged the USB keyboard, it still had the problem. - The problem only appears in Iceweasel, in the URL bar or in text fields, but not in other applications. - The problem went away after closing Iceweasel and reopening it. - I can find no pattern in when it appears. How can I determine the cause? Could this be the result of a security issue? I am using Debian 7.0 with a Logitech K120 keyboard.