This is probably the same problem as reported here:
Latest Event Added Shows for ALL Events on Calendar View
but it looks like it was never resolved.
My stats:
The Events Calendar 3.9.1 (Not PRO)
WooCommerce Tickets 3.9
WooCommerce 2.2.1
WordPress 4.1
I noticed at some point that certain events titles in my calendar would get over-written by any new event title that I created. The event title of the latest event that I created would replace several existing event titles in the calendar. When the event title was hovered over, however, the “true” event title would appear.
You can see it in action here:
http://parkeastdayschool.org/calendar/category/pe-events/
(For events January 30, 31 and February 1, also November 25, Dec. 16, 2014)
I haven’t found a pattern as to why these specific dates keep getting over-written (it’s not all events, just these.) Although they ALWAYS get overwritten by the title of any new event I create.
I did some troubleshooting and discovered the problem disappeared when I disabled my WooCommerce plugin. I tested also disabling all other plugins and switching to a default theme. WooCommerce was the trouble. I need WooCommerce on to run the e-commerce part of the site. This looks like a bug.