I am trying to figure out one thing in IntelliJ IDEA 10:
Press Ctrl + Alt + Insert to disable the column selection mode.
Please note that this effect can also occur if you activate Column Selection Mode under: Edit -> Column Selection Mode which allows you to make square selections.
File
| Settings
(or Preferences
on Mac) | Editor
| Allow placement of caret after end of line:
You can set the Caret behaviour as below shown in the screenshot: