If possible, I'd try to remove that one square and reinsert it with the right side up. If there's too much quilting over it and it's not possible, then I'd applique another square overtop of it, then "mimic" the quilting that was on the square by not going all the way through the entire sandwich, only the top layer and batting. with the new quilting.

I would either applique over the block with the same fabric or try and unsew the interfacing block and flip it over to the right side. Good luck and so sorry this happened to you. You definitely are not alone in this problem. I think we all have have something like this happen to us all.

We all sympathize.

If it wasn't for the interfacing and the different thread catching, etc. issues, I'd leave it as is. But since this is a memory quilt and is for someone else, yeah... you have to do something.

Label is good, applique is good, taking out and redoing is pretty awful but is good in the end.

My story of such a thing was a storm at sea quilt, all different pieces of blue. I don't know how many times I handled each block, dozens I'm sure as I pinned and sewed and pressed and quilted and bound... But I didn't notice one of the larger squares of fabric was inside out until it was hanging in a show. No one else ever commented on it and I left it as it was despite my desire to do something with it. It is indeed a humility block, I've never ever had to put in a deliberate mistake to any of my projects, there's always something that gets in and that I have to deal with in one way or another.

I agree with those who say the best solution is to pick out all the stitches and put it back together right, but if that's too difficult, I wanted to point out that iron-on interfacing will usually come off if you heat it as you did when you applied it. If you can test it first on a scrap of the same fabric. There might be glue residue. If it's t-shirt fabric usually it's the same on both sides, but I'm guessing you have some sort of design that is on the inside now, and taking off the interfacing won't solve that problem.

I have lots (!!!) of experience picking out stitches and want to say that if you don't have good lighting and good magnification and a very sharp seam ripper, now would be a good time to get them. Ott makes more than one version, depending on your budget, so that's one option, but I have been using some Mag-Eyes next to a bright window when the sun is on the other side of the house to avoid glare. You'll be surprised how much easier it is than the same work in ordinary indoor light without the magnifier. Do pick out the quilting from the back because if you slip and make a hole there, an applique is a better option than on the front. I have one where I appliqued a piece of fabric over a hole, and because the backing is a busy print and I was able to line it up, I would have a very hard time finding it even though I know it's there. Good luck!

Editing to add that if you want to match the quilting exactly, try tracing or drawing it before you rip it out.

Taking off the interfacing with heat will not solve the problem. The interfacing is attached to the wrong side of the fabric, so the wrong side is still up. It needs to be flipped to right side up - - - somehow. Or fixed some other way.

I have no other suggestions beyond the ones already given. I just wanted to add a word...or two.., We all have been there with mistakes, big ones and little ones, so we sympathize with you on this dilly of a mistake. You'll feel a lot better if you fix it the right way and only you can decide which way to do it. IMHO, that's not the easy way. I think you will just have to take that mistake out and flip it, or take it out and replace it with a new one, if you have more ties.

Remember that you have friends here who wish you well and know you can fix it!! Just grab a cup of tea or coffee, take a short walk and start working on it! Keep us posted on your progress and show us some photos!

I would pick out the stitches and correct the mistake.

Or, if you could photocopy a picture of his grandfather onto fabric and applique it over the block, that might work.

Aha! A picture of Grandpa on the quilt in his dorm room might be a bit of a deterrent to any hanky panky in that room! :D

>>I wanted to point out that iron-on interfacing will usually come off if you heat it as you did when you applied it. If you can test it first on a scrap of the same fabric. There might be glue residue.<<

I'm late to this particular topic, but if you do decide to leave it, and need to get rid of glue residue, you might try Goo Gone, which does get rid of the sticky left from glued on labels, etc.

Ditto what others have said - - either fix the faux pas or applique over it.