AmyMay
Situation normal
The OP is a human being, not an automaton - although it is unwise to do such a thing on a horse, it is not a cardinal sin, but was a natural consequence of being taken by surprise! I have an extreme startle response - when I am concentrating, I will half jump out of my skin and perhaps not shriek, but go "oh" rather loudly if someone comes up behind me unnoticed by me and says something.
No, it's not a cardinal sin - but rather an over reaction, don't you think?