(thanks go to my partner in crime, jorendorff, for helping flesh this out.)

lookdict calls PyObject_RichCompareBool without using INCREF/DECREF on
the key passed.  It's possible for the comparison to delete the key from
the dict, causing its own argument to be deallocated.  This can lead to
bogus results or a crash.

A custom type with its own __eq__ method will implicitly INCREF the key
when passing it as an argument, which prevents incorrect behaviour from
manifesting.  There are a couple ways around this though, as shown in my
attachment.