Just heard about a nasty little bug in the April CU from my friend Syed. He was using the SPWebApplication.MigrateUsers method to migrate accounts from one claim value to another (i.e. like if you were migrating from Windows claims to SAML claims, or in his case, changing identity claim values). Turns out after doing the migration the actual claim value in the content databases was not getting updated. He managed to track this down to a bug in the April CU, but found that it was fixed again in the June CU. So…if you are doing any kind of claims migrations – including updating an identity claim value – make sure that you are NOT on the April CU, but instead apply one after that.
Thanks Syed for passing this info along.