We're having this problem across all of our sites since migrating to SP2010. Occasionally (let's say 1/50 times), when you click the Calendar icon on a date, instead of seeing a calendar, you see the word "true" written where a calendar should be. The area that would normally occupy a calendar is white, suggesting that a div tag has been added to the page, but there is no calendar. You cannot dismiss the fake-calendar. You have to refresh the page. Below is an image of the problem. We can't determine any sort of pattern or correlation. I haven't been able to find anyone else having this problem online when I searched.
Has anyone else experienced this? Have you come up with a solution or figured out a cause?