Why is the minimize window function disabled sometimes?

Once in a while, the yellow minimize button on some windows is greyed out, and the Window → Minimize menu entry is likewise disabled. Right now, for example, this is the case with two open Terminal windows. If I open a new Terminal window, it can be minimized, but the old ones remain unminimizable (if that's a word). I tend to think of this as a bug. Or is it a feature? If so, what causes it, and what can I do to enable minimization of these windows?

There was an old QuickTime bug that caused the minimise button to become disabled on some windows.

But also be aware the app can disable the minimise button itself.