Carpenter's Square sofa quilt in greens, about three and a half years ago. It turned out nicely and my partner is particularly fond of it, but things I wish I'd known before I started:
1) My current, quicker method of piecing, learnt from that queen of hand-quilters, Jinny Beyer (well, her book). Those blocks took 6 hours each at the speed I was going at back then, including marking and cutting. And I only discovered audiobooks about halfway through!
2) Which way to point a rotary cutter (think I gave up and used scissors).
3) That it's a lovely pattern for a quilt where you'll see the whole thing at once, but looks a bit boring when it's scrumpled up on a sofa and you can only see bits.
4) Most importantly, that the woman at the fabric shop who told me that you only need three fabrics for a quilt was talking UTTER NONSENSE, at least if you want the quilt to be interesting. I use two dark greens which look practically identical, a medium green, a cream, and ended up adding a green batik for the border. The other fabrics were all semi-plains, so really not that interesting visually.
5) Also not to trust photos in catalogues or online, and to get some of the fabric before you commit yourself. The quilt is a smidgen too olivey for the sofa, though I just about get away with it.