So I discovered this bug while playing with a shard golem deck. (a long while back)
I was trying to burrow my immortal golem so that I could try to SoW the golem then unburrow it to get a total of +8 dmg.
Needless to say, I found out that I could SoW the golem while it was burrowed.
But then I couldn't SoW the golem even after it was unburrowed.
Later, I did some more investigating on this bug. Here are some things I found:
So, when a creature is immortal,
burrow it --> can not be targeted, period.
unburrow it --> still has immortal mark, but is essentially mortal.
Can no longer cast SoW on it.
When a creature is immortal AND is SoWed,
(does spell dmg)
burrow it --> can not be targeted, period. Still does spell dmg and has SoW mark.
unburrow it --> still has immortal mark, can be targeted, but not by SoW. But STILL does spell damage