In March of this year (2010) I inspected a 99 Boxster (with 70K miles) for misfire codes on cylinder #3. I performed a leakdown on #3 cylinder and obtained a value of 90%. I then borescoped the cylinder and found the beginnings of the D-chunk failure. Shown below are three pictures that I took of this impending failure.
The top of the chamber is at the bottom of the picture. The ring travel is to approximately the horizontal line. The crack has started from the very rim of the cylinder bore and proceeded downward to the top of the ring travel.
In the next picture, the other crack is shown below the upper limit of the ring travel. Note the seize mark of the piston (or ring) is in line with the crack.
In hte next picture both cracks are shown. In the terminal phase the cracks will start to connect to each other laterally and the resultant chunk of cylinder bore will fall into the cylinder usually while the engine is running.
The moral of this story (if there is one) is to stay vigilant on the service of the engine. This engine had a long history of misfire codes where the various service centers had just replaced the spark plug(s). No one had pursued the underlying problem of why the plug was continuing to foul. This repair will necessitate a factory crate motor which is probably more than the value of the car.