Trainers will tell you, "Management always fails," and you will think they are wrong. You will think you can manage the environment to prevent your dog from getting in over her head. You are wrong.
Management fails when the electrician shows up quite early so your fearful/reactive dog is not yet confined, and you are slow responding to the doorbell.
Then the siding guy outside tells the electrician you are home with 2 sick kids and are probably busy, but to go ahead in through the garage, out of the rain.
Then your dog who is reactive to some men is there before you are because she is a dog and you are a mere mortal.
Then the electrician leans over to remove his shoes, keeping steady eye contact with the dog who is barking and moving back and forth toward him and away from him.
Then he stands as you hurry forward, reaching to shake your hand. In that series of moments that lasted just seconds but felt like an hour, management failed.
That lean, that eye contact, that reach toward mom would be enough for many dogs to have lunged forward and bitten. In this situation, I called her away, asked him to hold on and stop looking at her, then got her a kong and closed her into a room out of the way.
My Australian Shepherd mix, Bella, is a dog with a spotty history before moving in with me. She retreats from triggers, typically, and we have done so much counterconditioning/ desensitization around the guys who work around my house from time to time, but never enough to generalize to everyone. She has been so relaxed around so many male strangers for so long but, once in a while, there is one that either looks or smells wrong to her. I suspect she remembers something from her life before that has a very negative association. I wish I could pinpoint exactly what it is to fix it definitively. Luckily I have a dog who I have trained to have a strong recall amid distractions, even when her fear has overtaken her.
Have a back up plan.
Stay calm.
Do the CC/DS.
Do the recall work.
Have stuffed kongs in the freezer, ready to roll.
Management always fails, even for trainers.