In general I have a difficult time knowing when to defend my position that a mistake was not my fault. I'm working in a retail store right now, but I've had this question in other environments.
Where I work there used to be only one person working at a time. Recently it switched to two. There's certain tasks that must get done and these are shared responsibilities. My boss messaged me in the morning that I have to write yesterday's report as we forgot to write it yesterday. We agreed he would do it while I worked on other tasks, and I even saw him writing it. I'm guessing he forgot to hit save. Should I message my boss this or just say I will take care of it?
Another time we were closing up the shop. My coworker had locked one set of doors and left the keys in the middle. Not knowing he hadn't put them back, I locked the second set. This was a huge problem as the person the next morning was locked out. I told my boss I will double check the key is where it belongs before locking the second door, but IMHO my coworker was more at fault for leaving it where it doesn't belong. Should I have said any of this?
Also the boss complained the store was left in a messy state. My coworker did not help with cleaning at all and went home early. I told my boss I had stayed until the end of my shift and done as much work as I could. Was this right?
Obviously I don't want to snitch or create an environment where people are trying to throw each other under the bus to get ahead.