neurtrial parking

What is your question?

hi today i went to the bank and there was a long line ahead of me and no cars were moving at the time so i put my car in neurtrial.And soon as i did that my car began to go in reverse.later i found out that my car was on a slope.the street was not flat it was kinda upwards like going up a hill.later when put my car in neurtrial on a flat surface it was find.Is this normal?