As the title says: an invalid delta in block_block_configure causes the following error:
Fatal error: Unsupported operand types in C:\Documents and Settings\PixelSmash\My Documents\xampp\htdocs\drupal\modules\block\block.module on line 507
To reproduce, go to admin/structure/block/manage/block/<unknown delta>/configure
While during most operations, this could never happen, there are no doubt certain edge cases where it could: I've found it because I created a block, added the Configure Block to the shortcut set, then removed the block. The delta in use by that block is then no longer in the database, yet the shortcut remains.