whenever our visas come back, I always check that our passport numbers are correct, that our names are spelled correctly and that we have a 6 month mulitple entry visa.
Last year on Tripadvisor VFS had stuck someone's OH's visa in their passport and vice versa.
She had to get a letter from HCI and at immigration they were pulled to one side whilst a discussion whether to allow them in went on before they were allowed through.
I also remember from a few years ago that someone had a total stranger's visa stuck into their passport.

Very recently on TA, someone posted that although her OH's visa was attached to his passport, it hadn't been signed by a visa officer or stamped with the issuing IHC office seal which is normally found on the bottom right of the visa.
After emailing VFS, he was told that the passport had to be sent back to VFS at Hayes with a covering letter to be then sent on to the IHC for signing and stamping. They also had to supply 2 SDE's at their own expense.
So, just to be sure, check all details thoroughly as soon as you get your visa back.