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I see no harm in your example. Even though the tuple has
NULLs in it, is in a stable state. The lesson learned is
that an object should become gc-tracked only if it is in a
stable state, as gc-tracking means to expose references to
the object. This is true independent of get_referrers, as
gc-tracking means that tp_traverse might be called for the
object, so it *has* to be in a stable state.

I fail to see how the example "crashes" the Python
interpreter. I causes a SystemError when the tuple is
resized, that's all. There are many ways to cause a
SystemError, including

raise SystemError

I recommend not to declare a function as "dangerous" in the
docs. Instead, the actual problem should be explained, both
in the GC part of the C API, and in gc module docs (for both
get_referrers, and get_objects).