ANSI Common Lisp   3 Evaluation and Compilation   3.5 Error Checking in Function Calls   3.5.1 Argument Mismatch Detection

3.5.1.7 Destructuring Mismatch

When matching a destructuring lambda list against a form, the pattern and the form must have compatible tree structure, as described in Section 3.4.4 Macro Lambda Lists.

Otherwise, in a safe call, an error of type program-error must be signaled; and in an unsafe call the situation has undefined consequences.