These things happen but we get more used to the different models and makers we settle in and hopefully these problems are less likely to happen. I'm always amazed at how long a TMC 100 is for the size, for example, but I'm not caught unaware. If I always used tiemcos it wouldn't even matter unless converting from someone else's size using different hooks. Except in lots of literature you can see insects and other fish foods described by hook sizes, a good example is Pat O'Reilly's Matching the Hatch. It's a good book, but it still leaves my mate Stevie in danger of stocking a box with flies that are considerably off in size. Which can make all the difference. The same is true when planning a trip to a far flung destination, whether fresh or salt, you'll often see recommendations of flies listed in hook size. Admittedly it's not often a huge problem, but people do end up in places with unsuitable flies (and gear) often enough for outfitters to be making videos, podcasts and information packs about it.
So what's the answer? Well for imitative tying I think there's not much to beat a good field guide that lists species with a size in milimeters Malcom Greenhalgh's book Trout and Grayling Food is a nice one. And I recently stumbled across www.rakkenes.com a Norwegian site with some beautiful photos of insects and descriptions of sizes along with suggested imitations-ideal for beginners and more advanced anglers alike. Of course you still need to know which size of your preferred model hook you need, but 7mm is 7mm whether it's a 14,16 or 18. Similarly 15 cm is 15 cm whether 1/0, 3/0 or 5/0
Maybe the mistakes are all part of the process though, but I sometimes wonder if there are unnecessary pitfalls awaiting newer anglers that could be got rid of.