Another timing Question

I was setting the timing on my '66 289 and just about had it where I wanted it and then all of a sudden the timing jumped by about +30 degrees. Nothing really changed in the way it was running. Could the menchanical advance have stuck in the distributor? How would I check? Anything else that would cause this? I checked it with two seperate lights. I don't know of anything specific that I did. I was mildly revving the engine periodically, just checking to make sure it would come back to idle.

Mallory Unilight Distributor.

Thanks.