If you choose to make an event longer than 24 hours by 1 minute up to 59 minutes, the end time in the event editing box still shows the end time as 24 hours later (as does the title bar for that event box). Yet when you save it, the calendar shows the correct end time. In fact, you can "predict" this will happen, because until you change the end time to at least 25 hours later, the event length box stays at "1 day". From 25 hours on, it seems to behave correctly.
So for example, create an event with a start time of 6:00 AM today, and an end time of 6:30 AM tomorrow. It looks fine in the calendar, but when you open the event, the end time shows tomorrow at 6:00 AM.