When you mouse over line numbers in IDEs, such as Visual Studio or Notepad++, the mouse cursor is flipped horizontally. I can't think of a reason this would be a ubiquitous user interface decision. Line numbers are already set apart from the code by colors or lines, and the cursor flip doesn't prevent the obstruction of the numbers, or make setting breakpoints easier. Does anyone know why this is a common decision?
The first time I saw this behavior was with Office 95. Word, to this day, still does this.
In Word and Other applications like Notepad++, this cursor change indicates "a whole line" is the point of selection. Clicking while the cursor is in this state will select the entire line.
Selecting an entire line is especially useful activity in text editors like Notepad++ when the line can end beyond the right portion of the screen. Clicking and dragging in this space will select multiple lines. Changing the cursor gives the user a cue that some different action can be made. This invitation to click is non-destructive and explorable.
On a personal note, when the cursor does not change (e.g. Eclipse IDE), I'm frequently disappointed by the lack of this feature.