OK thanks for the tips fellas. I shut off the flame while adding the LME, but never took the boil bucket off the burner. I reckon you are right that the hot supports could have easily scorched the syrup as I poured it in, even with flame off.
Question for unionrdr: Most recipes call for bittering hops at the beginning of the boil. If we add LME or DME at the end and hold temperatures up for a while to pasteurize, does this increase bitterness, or does the bittering only take place when the wort is on a boil? In other words, do we need to compensate by adding hops later (or using less of them)?
As I said, I am a noob. I use a wort chiller, so my temps are not hot for long after the boil is finished, so I have no experience with keeping the wort hot following a boil.