Doug Ellison, an imaging guru who happens to work at JPL, quickly told me in a Twitter update that the bright spot is due to a "cosmic ray hit" affecting the rover. (Later: The Surrey Space Center's Chris Bridges agrees.)
Update for 12:50 a.m. ET April 8: Here's a recap of the four pictures we're talking about:
The NASA team using data from Hubble Space Telescope has weighed the largest known galaxy cluster in the distant universe, catalogued as ACT-CL J0102-4915, but its nickname is El Gordo which means "the fat one" in Spanish.