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I did some maintenance work on a fairly large C++ system that called into a custom lower level library written in C. Much of my work involved extending that lower level library. It processed several billion dollars a year in financial transactions, and was barely documented at all. It was intended to be "portable" so it had bizarre #ifdefs all over the place in case you happened to compile it on an ancient Unix with a K&R compiler from 1986. This was the early 2000's, so nobody ever did that. I doubt it would've worked, yet those #ifdefs remained "just in case."

The lower level C library was pretty clever. The original developer of most of it left about a year into my tenure there. He was one of the smartest guys I ever worked with.

The C++ "app" layer was a different story. The worst part it was a 3000 line switch/case block with about 100 different cases, chock full of copy-and-pasted code. It went on... and on... and on... I still have nightmares about it.

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Type Opcode is ( NOP, Add_A, SUB_A, ..., Rem_D ); Procedure Execute_Instruction( State : in out Machine_State; Instructions : in Instruction_Stream ) is Subtype A_Series is Opcode range Add_A..Sub_A; Subtype B_Series is Opcode range Add_B..Sub_B; Subtype C_Series is Opcode range Add_C..Sub_C; Subtype D_Series is Opcode range Add_D..Sub_D; Procedure Do_Add_A; -- other subprograms. Current : Opcode renames Decode( Next_Token( Instructions ) ); --... Begin Case Current is when A_Series => case A_Series'(Current) is when Add_A => Do_Add_A; end case; -- other series. end case; End Execute_Instruction;