I recently mentioned in a comment that acceleration (from standstill to almost maximum practical off-road speed) is a much more interesting spec of tanks than their top speed, and has been so since the 1930's. Cruise speed on a road without excessive wear would be another very interesting spec, as are maintenance and repair times and pat durability (such as durability of track segments in km).
The typical data sheets don't have this kind of information, though - and thus we cannot tell much by looking up such superficial information as dimensions, top speed on roads or on-road endurance.
There's another very interesting example, and it does bug air forces (and navies*) even as of today:
The trouble caused to infrared guidance by the sun.
Infrared seeking missiles did in their most primitive type merely correct their stabilised flight towards a source of thermal radiation (a hot spot) in their field of view. The earliest applications involved experiments with teaching bombs to fall onto ships or smelting works.** A relatively new (late 80's in-service) one involves anti-tank missiles.
The classic approach is to use such a passive IR guidance against aircraft, though.
The sun does cause problems to this; it's severely hot, and large, and IR guided missiles tend to be unable to aim at a target that's within an approx. 5-15° angle to the sun.
Early surface-to-air missiles with IR guidance were only looking for "hot", not for "warm" targets, and were thus only able to lock on the exhaust nozzles of aircraft. This meant they were only able to be fired at departing aircraft, not at aircraft attacking something close to the launcher. This was bad because this way the aircraft's speed was subtracted instead of added to the missile, reducing its effective range very much.
But consider this: The attacking pilot was able to execute his attack run safely and then withdraw towards the sun - again "safely".
There are plenty remarks about how IR missiles such as Redeye, SA-7 or Chapparal were unable to attack head-on, but when did you hear or read about the problem with the sun?
So that's an example of how even when "hidden values" are described in addition to spec tables, you might still miss out on critical info.
What about Stinger et al? Well, Stinger can in theory attack head-on (still difficult, especially due to early warning and IFF issues), but what if the attacker approaches with the sun in his back? Again, even Stinger et al would be restricted to play tailchaser, with an according reduction of the effective engagement envelope.***
And other times, when they were launched nicely, the pilot may still have the opportunity to move between missile and the sun.
Last but not least, a pilot could approach (for attack or recce) with the sun in the back, turn within a few seconds and withdraw between missile and sun.
(The challenge is in all these attack patterns to guess the launcher position correctly and to not face many dispersed launchers, albeit they could be countered by staying above their service ceiling most of the time.)
In the end, the time of day and missile technology in use might be used to predict the favoured direction of air attack!
This kind of hidden values explains why the military is largely a technocracy, with relatively little successful interference by politicians in regard to tactics and technology. It also means that the military bureaucracy can hide impotence and incompetence fairly easily until it flunks the live fire test of warfare.
Oversight is thus not only difficult, but also especially important. Maybe testing establishments should report not to the ministry of war (defence), but to the parliament, similar to accounting oversight agencies?
S O
*: Rolling Airframe Missile (RIM-116) against sea skimmer missile at dawn, for example.
**: You might find this document about an IR seeking Japanese WW2 missile interesting. It's rare information on largely unknown Japanese PGM technology. Germany had similar developments, and IIRC the British and Americans as well
***: In case you think those optic filters solved the issue; at least the early Stingers were still vulnerable: "If IR cannot be obtained, the seeker may be locking on the background instead of the target. The sun is an extremely strong source of IR radiation and the seeker may lock on it instead of the target. The sun's IR radiation is also reflected from objects, causing these objects to become secondary sources of background radiation. When the target is approaching through clouds, haze, close to the ground, or is between the gunner and the sun, background lock may occur." link.
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**: You might find this document about an IR seeking Japanese WW2 missile interesting. It's rare information on largely unknown Japanese PGM technology. Germany had similar developments, and IIRC the British and Americans as well
***: In case you think those optic filters solved the issue; at least the early Stingers were still vulnerable: "If IR cannot be obtained, the seeker may be locking on the background instead of the target. The sun is an extremely strong source of IR radiation and the seeker may lock on it instead of the target. The sun's IR radiation is also reflected from objects, causing these objects to become secondary sources of background radiation. When the target is approaching through clouds, haze, close to the ground, or is between the gunner and the sun, background lock may occur." link.
I remember reading about military hardware as a kid and seeing "this thing had poor X,Y, and Z, but was used with great success on the Q front, where it was popular with troops." I didn't really understand at the time, the more I read when I was older, the more it became obvious how much the little design choices were as important as the big obvious ones. It probably started me on my interest in logistics. Ease of service and reliability of the weakest part is one of those things they never talk about on stat sheets either.
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