What is the only condition that would cause the Insert operation of a dynamic data structure to return a “data structure full” error?
I think this belongs more to StackOverflow than here.

As for your question, nothing in reality is unlimited. Computers have finite memory, finite storage. Words have a fixed (usually 32 or 64-bit) length, which might overflow. Or there might be some other engineering decision that limits the size of these data structures. Unlimited anything only exist in textbooks.