An alternator could be severely damaged mechanically in attempting to sychronize it with the power line under what two conditions could this happen?

I'm not sure this qualifies as two different conditions, they are both similar.

1. If the generator is more than a few degrees out of phase.
2. If the generator phase angle is leading at the exact moment it closes to the bus.

Explanation for #2: When manually syncing a generator, it is quite difficult to close the breaker at exactly 0 degrees, which is, of course, the desired outcome. If you close the breaker with the generator lagging a few degrees, the bus voltage (utility or other generator) will attempt to speed up the generator as if it were starting a motor.

No harm done if the phase difference is just a few degrees. If, on the other hand, you close the breaker while the generator is leading, the bus voltage attempts to 'yank' the generator back to where it belongs, like a brake. If the phase difference is great enough, this can shear driveplates and even break crankshafts!

The operator, then, shoots for zero to a couple degrees lagging when the breaker closes.