#18518: mention that including a return statement changes/breaks the behaviour

Doc/library/timeit.rst

@@ -63,6 +63,12 @@ The module defines three convenience functions and a public class:
    Create a :class:`Timer` instance with the given statement, *setup* code and
    *timer* function and run its :meth:`.timeit` method with *number* executions.
 
+   .. note::
+
+        Because :meth:`.timeit` is executing *stmt*, placing a return statement
+        in *stmt* will prevent :meth:`.timeit` from returning execution time.
+        It will instead return the data specified by your return statement.
+
 
 .. function:: repeat(stmt='pass', setup='pass', timer=<default timer>, repeat=3, number=1000000)