I have/had this problem, too.

You probably merged contacts from various "sources" -- from addressbook, skype, google, etc. and have birthday set more than once. There is also another bug that appear if you try edit such contact -- it will not save changes.

My solution -- unmerge problematic contact, for each part* check details and start merging again. Sometimes you need to remove birthday detail from one part, but you will get it back from other. If wont't save after merge, try merging one by one. Very annoying.

I hope it's clear

Cheers,
b.

*part = contact source, for ie. contacts, skype, google, fb ...