Theoretically, you could overspeed the turbo in the P-38 or the P-47, but you had to be way up high, and really moving to do it. Most often, it happened to P-38's, when the pilots were not properly instructed on how to operate the engines. Simply put, they let the oil get too cold, causing it to congeal in the turbo regulators, allowing the turbo to overspeed and explode. There is an armored ring around the turbo to protect the rest of the plane from being damaged by a turbo explosion.
When WEP includes water injection, or water/methanol injection, yes, an engine could be quickly and easily severely damaged if the power setting was used after the water or water/methanol ran out. The water or water/methanol is injected to lower both exhaust and combustion temperatures to prevent detonation. To continue to operate an engine under detonation WILL destroy it.
Detonation will:
Crush ring lands on the pistons.
Shatter rings.
Break or burn valves.
Hammer out rod bearings.
Melt pistons until they cave in.
For dry WEP, it simply overheats the engine. Power will be reduced, and if the throttle remains in the WEP setting, the engine will fail.
You can see all sorts of examples of these failures if you see the air races at Reno. The engine failures seen there, and seen often, are the result of operating the engine at WEP type settings and having it fail due to overheat and over stress, or failure to properly manage "wet" WEP.
You'll hear Reno racers talking about the "tweedle" valve, or "tweedling" water. In MOST planes, the pilot has to monitor the engine temperatures while flying at nearly 500MPH 20 feet off another plane and 50 feet off the ground, and "tweedle" water to keep the engine temperatures within the correct range. Too much water and it makes less power, too little and it blows up. It costs about $100K to fix one, if it can be fixed. The top teams, Dago Red for example, have on board telemetry, that broadcasts the engine parameters back to the crew on the ground. They radio the pilot and tell him to "tweedle'' more or less water, or otherwise adjust to control the temperatures. They can also alert the pilot to impending failures, giving him more time to pull out and make it back to land.
An interesting note. The Merlin engine was unable to run hard enough to win at Reno and stull finish without catastrophic failure until someone adapted Allison connecting rods to the Merlin in the late 1960's or early 1970's.
