It definitely sounds like a compiler bug. Unless you can provide further
details to the specific error in the C code of Python, it's likely that
we can do little about it.

If you want to analyze this further, here is a number of things you can try:
- compile Python at various optimization levels. A compiler bug often
manifests itself only at a specific set of optimization flags.
- try tracing this invocation of .find() in a debugger. Doing so at a
lower optimization level is easier, since the compiler may have inlined
the various functions that form .find() under optimization.
- if the debugger does not allow to pinpoint the erroneous function, add
printf statements.
Most of the code to study is in Objects/stringobject.c and
Objects/stringlib/find.h.