When something doesn't come out as planned I have three phrases:
1) "Mrs Unaware, despite my best efforts..."
"...I couldn't completely close the gap so you'll have to floss really well"
"...I couldn't get the root tip out since it's locked in so deep and I don't want to do more harm than good by digging it out, so I've decided to leave it there"
"...the instrument created a small hole in the sinus, but I'm doing everything I can to close it up so it'll heal nicely"
"...I tried to avoid the nerve, but the cavity is so deep, the tooth needs a root canal procedure"
2) "Mrs Unaware, I got good news! And even better news! The good news is that I took out the cavity and put a brand new filling in there. The better news is that I think I can make this filling even better. I know it's a humbug, but I'd like to redo this filling at another appointment."
answers:
"Ok, Dr Hack, whatever you say"
"Why Dr Hack, what's wrong?"
responses:
"Well, I don't think it'll bond just right"
"Well, it looks like the band moved and now there's a slight gap between the teeth"
"Well, under this light it looks like the shade is different"
3) I use this one for crowns mostly: "Mrs Unaware, I'm a perfectionist, and I want you to have the best possible crown. Unfortunately I don't think that this is it...there's a problem with the margin/bite/whatever. I know it's a humbug, but I'd like to redo this again at no cost to you."
Moral of the story, don't put the blame on yourself OR the patient. And in some cases the blame is obvious on one or the other. In other words, it's like golf...it's never the swing, its the club's fault. Crappy swings are always the fault of a crappy clubs. Blame the material, the band, the light, the lab.
Phrase is in such a way that you're the hero and you're looking out for the patient (as you should be) no matter whose fault it is. Sweeten the deal with a gift (like a gift card) as someone mentioned.
Come off positive and upbeat. If you say "oh oh", "oh ****", or have the grim reaper look on your face, and shaking your head slowly, the patient KNOWS SOMETHING went wrong.
NEVER use the word FREE. Say "at no additional cost to you".
They know you're a human being. They may not expect perfection, but they deserve to know if the work is not up to par.
I'm realizing that I should go into patient management consulting. Maybe I'll write a book LOL.
EDIT: as DrJeff said the weak point will be at the margins. While etched but unbonded tooth will have some degree of adhesion, fluid will eventually leak, causing thermal sensitivity. If for whatever reason you couldn't communicate with the patient, and the patient shows up 2 weeks later and says it still hurts, you're off the hook with an explanation, just redo it.
EDIT: The most humbling thing for a patient to say is "Dr, I'm sorry to keep bothering you, but this tooth that you worked on still hurts" (whether or not its the operator's fault). They hold you in such a high regard, they think they're bothering YOU! I always tell patients "no problem, I'm here for you and I want you to be happy, so let me take a look at it".