There are following general restrictions:

§ The triangulation points and constraints must be projectable: a 2d CDT can intrinsically not triangulate points with identical xy-coordinates or recessing caves. Therefore, triangulation points with identical xy-coordinates will be removed during the triangulation process. The point with the highest z-coordinate is kept.  
hint General rule: the CDT can only “see” the xy-projection of the triangulation points and constraints in UCS. It has no height information during the triangulation process.             
hint Use the CC:POINTS:ELIM2D command to eliminate points with identical xy-coordinates before triangulating. This keeps your input data clean.

§ Constraints must not overlap and must not be self-intersecting. Constraints may have identical start or end points and may be collinear. However, they must not overlap, be self-intersecting or coincident: since the CDT operates in the UCS xy-plane only, degenerate constraints define over-determined points along their intersection (i.e. possibly different z-heights at the same xy-coordinate).   
hint Use the AutoCAD command _overkill to eliminate overlapping or coincident lines.

§ Boundaries must be closed and linear. The CDT only accepts closed linear polyline objects as boundaries. The polylines must exclusively consist of line segments.            
hint Use the AutoCAD command _decurve to linearize a polyline if it does not exclusively consist of line segments.

§ Boundaries must lie inside the convex hull of triangulation points and constraints. Boundaries allow defining arbitrary shaped convex and concave shaped holes, islands and bounds in the triangulated surface. This implies that boundaries can only be defined where a triangulated surface exists, precisely being the area inside the convex hull of triangulation points and constraints.              

§ A Delaunay triangulation is not unique over an evenly spaced rectangular raster. As a consequence, the direction of the diagonal in a raster may alter when triangulating identical point data twice depending on the insertion order of the triangulation points.      
Figure 36: Two valid Delaunay triangulations over a rectangular raster