For 22 years now, graphical OSes have offered the same puny little single ref-cell as the model of the clipboard, i.e., the storage for the copy-paste mechanism. Yet most of the interaction I have with my computer is stack-shaped; start performing a task, realize it requires a sub-task, perform the sub-task, and return to the previous task. If I happen to have copied something to the clipboard which I am pasting in various places and then need to perform a sub-task in the middle, this single register may become trashed in the process, with a "user-save register policy" as my only recourse.
I know Microsoft Office has some sort of fixed-width-but-bigger-than-1 array for its clipboard, but it's only within Office, not OS-wide.
I'm not saying the design of a stack-based clipboard would be obvious--for example, I don't know how the system should know when to pop an item off the stack--but a single ref cell isn't enough.