What design flaw are you talking about?
The kayfun series is a classic certainly, but if for a design flaw I find refilling it to be a pain:
Kayfun 3.1: it has a 90 degree angle problem with the refill hole; they tried to put it on the side but from below, and as a result there's a right angle bend you have to get past in order to refill; that's why the kayfun lite and many other
tanks put the fill hole under the
tank, which has it's own problem in that you then have to take the
tank off the mod in order to refill; the lemo makes an improvement in that part by not having to take the whole tank off.