If there's one golden rule to follow when making anything, it's "measure twice, cut once". You'll hear people world wide spreading this like gospel and for good reason. There's nothing worse than getting right to the deciding moment of a project only to find out that you measured something wrong.
Luckily, in the 21st century, I am fortunate enough to be working with digital design and a 3d printer, so discovering that I had made the internal support for my hat using the diameter as the radius, I was able to just cancel a print, change a couple of parameters in Solidworks and export the design again.
Sometimes though, human error isn't the contributing factor. This was the case for me, when the host computer decided it wanted to take a break from computing (it froze) mid-print. As a result, I was left with only a quarter of the support ring.
But, like all crummy situations, perseverance and a positive attitude got me through and I was able to use the unfinished print to at least ensure that my support would actually fit the hat it was due to reside in.
