This one has never happened to me. After looking at the code:
For some reason, it seems that your session variables are not cleared when logging out and in as a different user. There is an easy fix for this in the calendar code (one or two lines) that would make it difficult, however, to display personal events from two or more users in the future. That isn't too likely to happen, but I left the code as it is because it MIGHT need to happen.
So, I suggest you look into the session variables thing first. But I 'll make the fix anyway. Seems like a good idea.