There are two mistakes in cooling electronics that are more widely practiced than any other. Both are based on outdated industry standards, and they are equally dangerous. It would be hard to say which one is more popular—ask your colleagues and you will probably find that they subscribe to both of these myths.
One is that you can double the life (or reliability) of an electronic component for every 10°C you reduce its temperature. Read Chapter 30 of Hot Air Rises for why that is just flat out not true.
The second is the incredibly popular practice of using θja and θjc component thermal resistances to find junction temperature. That's just not right. Even the industry standard that defines these resistances tells you not to use them for that purpose. The following chapters chronicle my sporadic campaign to slap the wrists of those thermal engineers whom I find doing it, and tell them to cut it out.