Working with 1U rack servers, I suppose it could be an issue.
Anyone ever see the computer actually get warmer when opening the case?
Yes, definitely with 1U servers. They are designed very specifically to be pulling air in from one side and pushing it out the other. If you remove that constant airflow, all the heat can do is go
up, except
up is blocked off an inch or two away by the server above it. The heat ends up not going anywhere with only a little of it escaping out the front/back and a little more going into the server above (which is also bad). I remember working in 1U server design for a while and cooling was definitely one of our biggest issues by far given that our servers were designed to be very densely stackable.