I am a pretty good home mechanic with great workaround and diagnostic skills, but my wife's 2005 Jaguar XJ8 decided to teach me a lesson in humility:

While replacing the spark plugs and coils on my wife's 2005 Jaguar XJ8 "Bagheera", I noticed that the air filter had shrunk, leaving a gap at one end. The filter was pretty clean otherwise, so in the name of efficiency I decided to fix it. I cut the glue lines in a few strategic places, relieving the filter enough to be stretched. I then used a fiberglass rod from an old kite, sized perfectly and sharpened on both ends to brace the filter back to size. No more gaps to let nasty things into the intake. How clever.

About 4 days later, I happened to be driving the Jag with my wife and dog (actually on the way to pick up my 97 Jag "Baby" who got a new pair of shoes), and after a spirited on-ramping to the highway, "Bagheera" started throwing codes and revving erratically. I managed to take the next exit when I realized the power brakes were also out. Though it almost died, I managed to coax the poor Jag to a nearby Shell station and park safely out of the way. Eventually we all got towed home.

I suspected a major vacuum leak, based on the brake issue, but found no problems. When I tried to start the Jag, it immediately revved to the limiter then back down and died. Perhaps the throttle position sensor? I then removed the intake assembly and discovered my clever air filter brace missing! Much to my relief, it had lodged itself in the front of the intake manifold (too long to suck in any farther), where it had blocked the throttle plate from closing all the way, creating a 3 mm gap. Just enough to make "Bagheera" crazy. So much for keeping nasty things out of the engine!