I love that error message! That one was always my favorite, because of its length and complexity. It was like an error novella. We knew it was a problem, so it is gone now in 10.1. I'm not sure if it was service packed back to 10.0. I'll check that.
As for what it means, your output shape is set to "True Shape with Measures". This gives you not just the route shape, but linear referencing M values along the shape. For example, for a 10 minute long route, you can see where you are 2 minutes, 5 minutes, 7, minutes along, etc.
It used to be a problem when there was sudden additive cost along a route that should have M values. The error message lists "positive-impedance junctions, turns, or additive point barriers". Those are three examples of a place where you might add time to your route. For example, an additive cost barrier of 2 minutes turns your route from 10 to 12 total minutes. Previously, we couldn't handle generating Ms where, using the 10 minute route example, it went from 0 to 5 minutes, then had a 2 minutes additive point, then went from 7 to 12 minutes.
When additive costs were encountered, we still solved the route, but we disabled M values in the output route shape. A warning would be generated letting you know that, even though you specified that Ms should be in the output shape, they will not be present.
So, change your output shape to "True shape", instead of "True shape with measures." Then, you can turn warning messages back on.