The first thing that comes to mind for me is fabric. I can not embroider on Symphony broadcloth (cotton/poly blend from Joanne's). It always puckers. I think it's because it's such a tight weave and then the polyester doesn't allow the fabric to give at all. With 100% cotton you can steam out minor problems but not with this fabric!
The thing is that I like to use the fabric for costumes and other things because it washes and wears so well.

For me the most challenging were my daughter's wedding dresses.
The first one I made in '95. I had a Brother Galaxy with a limited number of embroidery cards, but I found one design I liked and embroidered it 41 times along the rounded edge of the removable trail that was attached at the waist. Unfortunately the embroidery is not visible at the picture.
The second one I made in 2000. I then had a Viking Designer1 and could embroider 150x240 mm designs. So I bought their card 102 and got stitching. I embroidered the cut-out parts before stitching the dress. I had made a muslin to get a proper fit and sketched in where I wanted the embroideries placed. She had gained a lot of weight due to over a year of a high dose of prednisone (and was very conscious of it) so the lines of the dress and the embroideries had to be as flattering as possible.
In both cases the price of the material (a heavy silk for the first and a satin for the second) as well as the occasion added to the stress, but the results were well worth the effort. The dresses took 100 - 120 hrs each including the petticoats, purses and in one case the hat.

I would have to say a project that is still waiting for me to begin. I have a hand embroidered quilt I am finishing. I am supposed to be making and embroidering the backing (42-15x15 inch squares), the dust ruffle and curtains. Motivation and being overwhelmed are my problems. That is 3 big projects all tied into one.

I'd have to say that making a waterproof cover for an airplane was quite a challenge. I had to take many trips to the hanger, to make the pattern & work with loads of heavy fabric. I also made seat covers & cases for portable oxygen tanks to match the upholstery. I embroidered the plane's call letters on everything.... had to get SOME fun out of this project!

I have a challenge for you. I will send you my darling hubby, who is sweet and loving BUT was the baby and only boy with 4 older sisters and you can re-train him. He can teach your boys guitar in exchange, because hey, that's all he wants to do anyway. :

My most challenging to date was my little Halloween bags made last October. You probably remember I could not finish the project as my machine decided to give up.

This was my most challenging project. It can be found at San Francisco Stitch Co. I embroidered the blocks and border. Pieced it all together then quilted it all with my embroidery machine. It is a crib size quilt when finished.

Any other ideas? Keep in mind I don't have a multi needle machine yet. I do love a challenge though.
Thanks to those that have offered ideas already.

If you remember the scripture quilt(look in projects) next to teaching myself Windows 8, that was the most difficult project I have produced. These days it seems just getting anything accomplished is a miracle to me. Much love Lillian

Not done, but on my own challenge to do list has the tile design wall hangings. I bought a few when they were on sale (Stained Glass-Tiffany's Guiding Angel, and a Hummingbird) - as it's "for me", they keep ending up at the bottom of the list (there are always gifts to be done for some reason or other). Now at the point of thinking I'll hold off on it until I get a new machine as I've been considering one. Egads - getting link and I see she's having a really good sale (53% off for a few more days) and I was just thinking I want one for the dining room...

currently, I am trying to embroider on little T-shirts. Not sure how to get the design on it without unpicking the side. Normally I make my own shirts, but this is for another person. Help would be appreciated!

My challenge is to cut 2 1/2 inch fabric strips (jelly roll). Mind you, I am not a sewer or quilter, I just use the embroidery machine and simple straight stitch. Jelly rolls pre-cut are not cheap, I have lots of fabric, so I thought I'd cut my own strips. It is a CHALLENGE. I have all the supplies - rotary cutter, mat, lge plastic ruler with a lip, etc.; but I've yet to really cut straight. Any suggestions are appreciated.

05 16 2015 It really was not embroidery, but it was sewing, and it was teaching my group of cub scouts how to hand sew.

Challenging for me is to deal with a thread nest. I rarely have them with my machine, but it always seems to happen when I am stitching on a customer personal item. There is no starting over and it has to be a save and I am screaming all the way. As many of you know if you don't catch it in time a broken needle or hole results. I even had the fabric get stuck in the throat plate and had to cut it out. Kay

I do hope other Cuties will provide some ideas for a good machine embroidery challenge for me to work on this summer.
I also hope there will be some to help the ones that have added challenges of their own.
Flowers for all

My challenge is to make a triangle banner string to fit approx 2 8' tables with blues, blacks, whites and grays. oh did i tell you there about 4 variations of each color...DO NOT PUT THEM CLOSE.. its REVERSIBLE....ugh thought i was doing good till i laid it out again. Now have to match it in with a second banner of blues and whites with letters embroidered into them single in between the wording. sew them into binding and hope to goodness it matches perfectly. BY NEXT SATURDAY IF POSSIBLE. anyone live by me...lol just hoping someone did.