Convection cooled equipment usually has a series of small holes drilled in the bottom of the case, usually in a square pattern, and another series of holes in the back or top of the case. This allows the heated air to escape from the top or back of the device, while at the same time drawing cooler air in from the bottom. A number of rack-mounted power amps utilize this cooling technique in conjuction with heat-sinks on the power transistors.
As you can envision, if the rack were mounted vertically, the heat would be trapped in the top of the unit, thus causing it to overheat. A classic example of this was with the Mackie 450s, which overheated if the powered speaker was placed in its side, but seemed to run just fine if it was placed normally. In order to overcome this, some owners installed 120-mm computer fans behind the heat-sink's cooling fins. The fans are extremely quiet and moved lots of air over the heat sink fins, thereby eliminating the problem.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!