Let's say you write a line of something go down but want to change the one on the top again... (Push enter type a script then press up to go to the line you were at last) and you start typing, it starts overwriting what you had before, or replaces the letters instead of moving them =\

Perhaps you pressed the INSERT key by accident? It toggles between replacing and inserting text in any editor.

Hmm. Maybe but it seems to happen more often depending on the size of the lines =\

The editor won't overwrite letters by itself, so it's most likely the INSERT key, as mentioned. When you run into that problem, try pressing it and see if it starts inserting the text again.