When first figuring out how to double flare brake lines, it took me forever to get one that didn't leak. After I flared both ends, I realized I forgot to put the threaded part on one end.
I did the one brake line over and over.
Now that I will admit to. Get the perfect flare, look down, and see that I forgot the flare nut.
The last time I was running brake pipe, I got the perfect flare, and then discovered I had the wrong nut. Cut it off, flared it, discovered I forgot the flare nut all together. Cut it off again. Third times a charm. :-D