When I'm learning a new stitch, technique, or crocheting a tension (US: gauge) swatch, things often don't turn out as planned. I don't call these pieces 'mistakes' but 'practice pieces' and keep them in a basket. Every few months I go through the assortment of practice pieces. I've found it useful not to do this too often or obsessively: each time I look through my practice pieces I come to them with fresh eyes & see potential & beauty, not 'something gone wrong'. For instance, most recently a small, too-tight swatch for the puff stitch beret became the backing for a brooch featuring a hitherto rejected first attempt at the spiral motif from the 'mesmerize' sweater. It made a lovely birthday gift.