"Severus!" Lupin scolded, trying hard to hide a grin of his own. "That was cruel!"
"Nonsense!" Snape insisted, wiping his eyes on his sleeves as he sat up again. "They learned a valuable lesson. You can't disregard your environment just because you're tracking."
Lupin rolled his eyes and shook his head, but Snape noticed he watched that part of the map carefully and shook with suppressed mirth every time another trio blundered into the house elf booby trap.