In a tenting failure, the ceramic tile usually comes clean off the adhesive bed, leaving an imprint of its backing. This is often misinterpreted to be a sign of a poor quality adhesive when it is actually a sign of a good quality, strong adhesive.
The concrete and tile adhesive both have cement as the binder and so they bond very well together. However, since ceramic tiles are made from clay, the bond between it and the cement in the adhesive is usually fractionally weaker. The cleavage plane or the point of failure will always occur at the weakest point in the bond which is why the tile comes off clean.
Another common type of failure is when the adhesive comes clean off the substrate and remains stuck to the tile.
Since the cleavage plane is at the weakest point in the bond, this is a clear indication of poor surface preparation (i.e. tiling onto a dusty floor or not applying a basecoat slurry to a steel floated floor).