I delivered a cake this morning that was an 8" choco and 6" strawberry.
I just remembered the customer had called me and changed her mind about the filling and requested strawberry bc on the 6" cake instead on vanilla bc.
I never wrote it down and completely forgot until now, I did'nt use strawberry bc I used vanilla. I double the original order confirmation before I filled the cakes and it said "reg bc", I had never updated it.
Is this a huge problem, should I call her and tell her I forgot and offer a partial refund or discount on her next order?
Its just the 6" and I'm sure they have eaten it by now and I have'nt heard anything from her?
I mean she still did get a strawberry cake with buttercream, just not strawberry butter cream.
Am I making a bigger deal out of this then it is?

I'd call and tell her that you remembered about the change, and offer a small refund or a credit on a future order. It's better in the long run if you acknowledge the mistake rather than waiting for her to call you.

I think I would call too. Tell her you are sorry that you forgot to note it in her file and you would be happy to offer a 10% discount off of her next cake for the confusion. I am sure that it didn't wreck the party, but she will be very happy to hear from you and will appreciate the jesture.