The concept of a “hidden intermediate” in a reaction pathway has been promoted by Dieter Cremer[cite]10.1021/ar900013p[/cite] and much invoked on this blog. When I used this term in a recent article of ours[cite]10.1021/jo401146k[/cite], a referee tried to object, saying it was not in common use in chemistry. The term clearly has an image problem. A colleague recently sent me an article to read (thanks Chris!) about isotope effects in the epoxidation of ethene[cite]10.1021/jo981652x[/cite] and there I discovered a nice example of hidden intermediates which I share with you now.