This can usually happen when are are picking geometry from an AutoCAD file. Revit will report this error when a wall is drawn at an angle at a with a fractional degree, such as 95.1 degrees. To correct the problem, either redraw the wall or change the angle. You can sometimes verify the angle by creating a 3-4 decimal point dimension style.
Special thanks to Daniel Hurtubise, aka the Revit Warnings Nazi for his help on documenting Revit warnings. Check out his RevitIt blog. (in French)