When an aircraft system is inoperative, the associated switches or push buttons are placarded with an INOP-sticker. Unfortunately, these small tags have black text on a red background. While that may seem conspicuous, the stickers are very difficult to see at night when the cockpit is dimly lit, aggravating the chances of operating an inoperative system inadvertently. A small piece of printer paper does wonders in such a case. Inexpensive and (very) low tech, but extremely effective. 🙂
