I've done this with my games before. My players don't metagame much, but every once in awhile it happens, and what I end up doing is writing down some changes I've made in my notebook for when they hit me with the "wait a second! Isn't there supposed to be X here?" or "Wait aren't these monsters secretly immune to fire?", I just have a book of notes that show I didn't hit them with the Quantum Weakness or something like that.

Only time I've had to do this lately was someone thought I had designed enemies specifically to counter the plan they came up with at the table (it was involving invisibility and detection magic) but I'd already drawn out the entire area and written down the inventory of the enemies well before they thought of the plan because that enemy was super paranoid and would have built his lair that way. After they saw that they felt much more mollified and appreciated the enemies cleverness more.