Civimail Mailings not saving or "corrupting"
This seems to be an issue wherein ckeditor does not emit a standard change event when using "Source" mode. Using a work-around from StackExchange, we get this proposed fix: https://github.com/civicrm/civicrm-core/pull/5869

We're very close the 4.6.3 release. With some luck, it might get into 4.6.3 -- otherwise, it'll go out with 4.6.4.