When we get servers into the shop to run up we normally have to install a number of additional components inside the beasts. This usually means that we rip off the cover and get everything installed and then power the sucker up to make sure that it all works. We also then generally leave the machine running so it can “burn in”, basically see if anything “burns out” and needs replacing before the customer takes delivery.
Recently, we left the side cover off the machine overnight and came back in the morning to discover a number of warnings being displayed in the server monitoring software. Turns out that the disks were running pretty damm hot. Why? Well, servers these days are specially crafted to ensure that they receive adequate air flow WITH the covers ON. Removing the cover completely destroyed the carefully crafted airflow designed to keep everything cool.
Once we returned the cover to the machine and switched it back on it was only a few minutes before everything was back operating at the correct temperature. So there you go, don't run servers for extended periods of time with the covers removed or you risk overheating the internal components.