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This is a duplicate of Issue #14010. This issue runs deep and isn't itertools specific. Running infinite chains of iterators pushes a number of CPython internals past their limits. The best solutions are hard (monitoring the C-stack or fixing every possible fault at the fracture point). The simplest solution (adding recursion count tracking to every possible iteration)is very heavy-handed and would slow-down all of Python in order to spare exotic cases that don't seem to arise in practice.