sewer line back fall
Critical... not likely, but if water is laying in the line, there is the chance that it has settled at a joint or even broken. The problem is probably a bad job of supporting the pipe when the trench was dug and back-filled. The water laying in the line will start impeding the flow, especially solids, and could cause a clog or back-up.
If you have never made a mistake, you haven't done much.