From https://platform.github.community/t/circular-references-in-fragments/314:

It seems that requests with circular references in fragments fail abruptly, instead of returning a validation error. See the "No fragment cycles4" validation rule.

True story! The query provided does refer onto itself, and shouldn't, because it results in a stack level too deep.

@rmosolgo Think you'll have time to look into this? If not I can pick it up, no worries. 👌