@evan I like "did you consider X?" as a conversation starter. If they did, then it's an easy conversation - if they did not, that's an opportunity for you to share why you think it's a better approach. If it's not too late for them to change their mind - and if it's too late, what's the value of informing them that they had another option? Is the point of your inquiry to communicate "If this comes up again, you might consider..." or "You fool, you should have..."

@evan Yes! One of my pet hates. First it assumes that the person you are talking to knows better, and actively chose not to do something your way. It also assumes that you know all of the constraints that the person has. And as you say it assumes that *your* way is better/easier/faster without providing evidence.