On a given line of the RStudio console or script writing area, the cursor as it flashes on the screen is shifted from its true position, where text is inserted. Say I want to delete a character on a given line. I click right in front of it, and the red cursor flashes where it should. But when I press backspace, the character six spots to the left gets deleted instead. This behaviour gets worse as the line goes on, so near the end of a long line, the offset is of 13 characters while it is only of 1 or 2 characters in the far left of the screen. I use Ubuntu Linux 12.10 (with MATE GUI).
The problem was in the RStudio font settings. I changed them all at once and the offset vanished. I changed the settings back to their original values one by one till the last, and the offset did not come back. RStudio support helped me through this.