If You See Something, Say Something
Don't be afraid to speak up when something just seems... off.
Maybe some metrics from a team dashboard look weird, maybe there's a change in a PR that you might not fully understand but feels like it might have bad consequences, maybe there's just some long standing code that looks like it can't possibly be right (but it must be if it's been in here for so long, right?). Often it's easy to assume that you must be misunderstanding things because if something was wrong someone else would have surely noticed.
Don't count on that.
Assume that everybody else is busy on other things and that maybe you're just the first person to look close enough notice that something isn't quite right. Maybe you really did misunderstand things. That's fine. Now you've learned something. But maybe you just caught something early before it became a big problem.