SpecificationError

A specification written with scalus.cardano.onchain.plutus.prelude.Spec does not hold. It is thrown off-chain only: a compiled script carries no specification.

So it is no OnchainError, which stands off-chain for a script that fails. A test that expects the script to reject a transaction does not pass because a specification was violated: the script, without the specification, may accept it. It is an AssertionError, as the one Predef.ensuring throws: what is wrong is the code or its caller, not the transaction.