Any time a receiver gets signals on its channel from two different sources, you have a problem. If you are flying a model in PCM mode, and someone else turns on a PPM transmitter on the same channel, your PCM receiver should go into lockout or failsafe, depending upon how it's set.

If the offending transmitter is on long enough, the model in the air will crash.

Only if the model could continue to fly, like a free-flight model, with the PCM transmitter actually turned off, could you say that the model woudn't crash.

We're sorry, but two or more transmitters on the same channel, operating at the same time, will effectively make that channel unusable for ANY flier, no matter what kind of coding they happen to be using.