Does a female cat still go in heat even when she is spayed?

I had that happen once that our female cat went inot heat AFTER being spayed. We went back to the vet & it turned out he had not taken everything out. He proformed the second operation for free. Then everything was fine. I do TNR trap neuter return so I deal with hundreds of cats a year and so far have never had this problem in my adult life. Both of my female cats show signs of heat affection, and my boys show signs of wanting to service them, although all of them are spayed and neutered. I guess you can take out the parts but not remove the instinct. Julie