One of the most distinctive types of coins of the middle ages, at least to me, are these bent ass Byzantine trachea. Alexios I reformed the coinage in the late 11th century CE, and for some reason, the empire started making most of them in what you usually see referred to as cup shaped or scyphate. We have no fucking idea why they did this. The best guess anyone has is that this was done to make them stack better, as pre-modern coinage usually lumped up quite a bit in the image and didn't stack well, but I've heard of collectors with a bunch trying that out and finding that their stacking ability isn't much better than other contemporary coins, and these three certain don't stay still there's much movement after I stack them.

Whatever the reason, they found it important enough to change their minting method to make sure they got this shape, and they kept doing it for a long time, with even the Latin occupiers continuing it after the 4th crusade. As you can see in the last picture, they had to awkwardly rock that top die around to get the whole image on the coin and to get the shape right, and it tended to not work very well. It's hard to line things up right when it's all wobbly like that. Like that middle coin above is badly double struck on the concave side (there's supposed to be one double circle around the figures, and you can clearly see two at the top of the image), and errors like that, incomplete images, and results just looking like muddled messes are common in bronze, though less common in gold (possible due to more care, possibly due to the softness of gold making things easier). And the way coins wear meant that whatever image you managed to get on the convex side could get obliterated due to wear fast.

It's utterly baffling to me that they went to this much effort when there are so many downsides and no clear meaningful benefit. That makes them fascinating little mysteries, even if the result of them isn't pretty by normal standards.