Position the cursor on a line of text with a line of similar length above (or below it). Then:
- Press down/up to move to other line and back to the first
- Press cmd+left to move to the beginning of the line
- Press down (/up) to move to the other line
At this point the cursor should be at the beginning of the other line, but instead it has returned to the original column.
A similar issue occurs on Linux after using ctrl+arrow to move between words, and with cmd+right to move to end of line on Mac.