Big ball of mud
A system with no discernible architecture, tangled dependencies, shared global state, and code organised by accident rather than design, where every change risks breaking something unrelated. The big ball of mud is the most common architecture in practice precisely because it's what emerges when structure is never deliberately maintained under delivery pressure.
The "big ball of mud" names the most common architecture in the wild: a system with no discernible structure, where everything depends on everything and a change in one place breaks another for no obvious reason. It's not usually incompetence. It's the accumulated result of expedient decisions under deadline pressure with no refactoring budget. Naming it matters because you can't argue for paying down structural debt that everyone pretends isn't there.