iCloud sync keeps deleting my information! Ever since I started syncing my Contacts and Calendars on my Mac with iCloud, I've noticed that stuff sporadically disappears.

I've had to re-add an event to my Calendar 3 times now, and many of my Contacts mysteriously vanish after a couple of days.

The only other device that syncs with iCloud is my iPhone, but I still lose events and contacts even if I don't make any changes on my iPhone.

What is going on, and how do I prevent this data loss?
Per Emil's suggestion, I contacted Apple's support team for assistance.  They recommended that I enter all my contacts into iCloud directly (via icloud.com), then delete my iCloud account from all my devices and finally set up iCloud for each device again.

It's been a few days, and so far, the problem seems to have gone away (: