Steps to reproduct: (http://www.daypilot.org/demo/Calendar/)
1. Select empty cell.
2. Right-click clean selection
3. Re-select that same cell (the cell won't be selected).
4. Select any other empty cell.
5. You can then re-select the previous cell.
Step 2 can be change to any other action (create an event, resize another event....), step 3 to 5 are still the same.
I'm using either IE 7 and FF 2/3 and the bug is still there.