When designs reverse their paper reputation

Which designs looked good on paper but failed in practice, or looked poor on paper and worked?

Béarn is suggested as a disappointing design, though its paper merits are disputed. The M-class, possibly the I-400 class and Pendennis Castle supply proposed examples of unexpected practical success.

Béarn's three differently sized aircraft lifts divide opinion over whether the carrier ever looked convincing on paper. One view treats them as an understandable attempt to test different aircraft, while the objection is that three large lifts could also handle the smaller machines. The defence of the design rests on the competing requirements accepted by its original designers. For the reverse case, the M-class submarines and, more tentatively, the I-400 class are proposed as unusual concepts that were reasonably well executed. Pendennis Castle supplies a land example: an imperfect star-fort layout could suit a position where the defences only needed to control the entrance to an isthmus.