Wasm is not quite a stack machine

Everyone knows Wasm is a stack machine. Wikipedia says so, the official Wasm design specification says so, you get it. I thought so too.

That is, until I started writing Wasm code – not compiling for Wasm, but writing the instructions by hand. And I found out that there exists a major difference between Wasm and all other stack-based languages, that makes this claim misleading.