回複：壓力山大， 請大家幫助解難， 多謝！

You need to tell anyway.  Most likely it is not as bad as you have thought.  If you are not the responsible person for the project, it is very unlikely they would make you be responsible for the mistake - because it would be too obvious. 
However, try to find if you have any documentation that shows that the decision is made by other people.   This is a lesson learned on having good documentation on everything.