I am running a linear static assembly simulation.  I've run other assemblies that are very similar (similar geometry, loads, contact sets, constraints, etc.) without a problem.  This is just another size.  As on the other models I've already successfully run, I have surfaces that I've set up contact sets between (with an initial gap) but I set the flag to "Ignore gaps" for those contact sets.  On the other models within this series, I haven't had any problems.  However, on this assembly I get the error below.  The error goes away when I deselect "Ignore gaps" and run the model.  The error seems to imply that I have shell elements somewhere but the part thicknesses and mesh size are small enough that I wouldn't think shell elements would be generated.

Any idea why I'm getting this error?

Is there any way to see where (or if) shells are being generated or should shells be obvious when looking at the meshed model?