cleric with THAC0 of -30?!

Hi. I've read through all 20 pages of postings and couldn't find anything on this topic; the 1.06 patch apparently doesn't mention it.

My cleric, who attained 8th level after completing Severed Hand (but not having returned to Kuldahar yet) suddenly has a THAC0 of -30! Is this a bad bug or no big deal? I'm afraid that too much negative might cause problems....

Notes:
All characters have been started and played normally through the game; no tweaking of game or characters.
None of the game patches have been applied.

The only things he equipped since entering Severed Hand (the negative THAC0 didn't exist previously... he was a positive 16) are the symbol of Corellon Larethian and Ring of Sanctuary.
I've attempted to de-equip him to nakedness, but when he's re-equipped, he's -30 again.

Any ideas about...
1) if this negative THAC0 is a bad thing?
2) how it happened?

Thanks in advance for your assistance.
Best regards --- js

PS: I just found that it's steadily increasing (or decreasing, depending on your POV). Now at 10th level, he's venturing Dorn's Deep with THAC0 of -100 (yes, negative one-hundred).

wow...that is ....well,,,,,wow....!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!I do not know, about that personally...I have not ever heard mention of it...I guess you are the special one !