Greg Meboe and Evan Pierce provided this pic of a dropped valve seat.  It is posted full size, so if you have a dial-up connection it will take a while to load.

It is the exhaust valve seat that slid here, which is fortunate; the exhaust valve is recessed in a pocket in the H.E. head, so it can drop quite a ways before the piston gets involved.  Unfortunately, the valve seat that decides to relocate can just as easily be the intake valve seat, which cannot move far at all before it holds the valve down far enough for the piston to hit it.  If the car continues to be driven with a dropped seat, typically the valve will beat the seat to pieces, after which the chunks will batter around and tear up the piston, head, both valves, and the cylinder liner.  Interestingly, the valves themselves never seem to break off under such treatment.

Nice view of what is obviously a Bosch Platinum +4 spark plug, too.

If you don't ever want to see this view of your V12, you need to learn how to read your temp gauge correctly.  On the pre-1992 XJ-S, the temp gauge is vertical with a C at the bottom, N in the center and H at the top (some are a little different, some even have a degree scale, but the gauge is the same).  If the needle is ever above the letter N -- even just a bit -- pull the car over, call the tow truck, and have it towed wherever it needs to be to have the radiator removed and either rodded out, recored, or replaced altogether before you turn the ignition key on again.

But wait!  Doesn't N mean Normal, and shouldn't the temp be OK until the needle gets up near the H somewhere?  Is the gauge inaccurate?  No, the gauge is accurate -- and yes, N is Normal.  The problem is that the sender for this gauge is in the right bank of the engine, and it's the left bank that has all the problems!  Thanks to an absurd 1-1/2-pass radiator scheme, any buildup of obstructions in the radiator affects the cooling of the left bank much more than the right.  If the cooling system is working properly, the temperature gauge will always read either on the N or below it (depending on the thermostats fitted).  If it is reading above the N -- even a hair -- it means the cooling system is not working properly.  And if the sender in the right bank is showing things are getting a bit warm, it is only too likely that the left bank is already severely overheated.  Stop driving now.

Other ideas: you can relocate the gauge sender to the left bank to get a better idea of the temp where it counts -- but it's not easy, requiring drilling and tapping.  You can even add a second sender and a switch and monitor both banks, switching from one to the other.  Or, as James E. Teston demonstrates, you can monitor them both constantly!

"This is tongue in cheek and it isn't a real dash, but you get the idea.  It wouldn't be too hard to do.  A generic amps gauge could be installed elsewhere ..."

The above discussion applies equally to the XJ12.  A SIII E-Type is also subject to valve seat droppage if overheated -- but there is no proclivity towards the left bank more than the right bank because the E-Type has a rationally-designed (if undersized!) radiator.  The 1992-on XJ-S and XJ12 have different gauges (round), but the owner is nevertheless advised to note where the gauge normally reads and take action as soon as it begins to run hotter than usual.