Degenerate inlinetasks are stated as allowed in the documentation at the beginning of org-inlinetask.el, but are maybe not so common since org-inlinetask-insert-task always inserts the END-line.
Since April, an infinite loop is caused when cycling an entry containing degenerate inlinetasks by commit:
5f184b org-inlinetask.el: Fix visibility cycling for inlinetasks
Sample for reproducing:
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* Heading
*************** Degenerate inlinetask
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Cycling "Heading" will call org-inlinetask-hide-tasks where the last calls in the loop of `inlinetask-goto-end` takes us to the line below the inlinetask, but `backward-char` takes us back to the point at the end of the inlinetask, so `inlinetask-at-task-p` is still t and the loop continues infinitely in this fashion.
I don’t have a good suggestion for a solution that would still solve the problem addressed in that commit.
Best,
Anders Johansson