Inoperative equipment and placards
Aircraft maintenance is responsible for ensuring all aspects of the aircraft meet the airworthiness & serviceability requirements. Sometimes they may not be aware there is a problem with certain equipment. Occasionally during a preflight inspection of the aircraft and equipment, flight attendants will find things that need to be addressed, such as missing weight restriction placards, tray tables that do not remain latched upright, emergency equipment that does not meet the preflight requirements, or something within a lavatory that makes it inoperative.
While on the ground, aircraft maintenance can and usually will address the problem and make it operable again. If the problem cannot be addressed in time, aircraft maintenance personnel will turn to the minimum equipment list and deferred the repair in accordance with applicable regulations, and they are supposed to apply the appropriate placard according to the minimum equipment list. Sometimes parts of the aircraft may break during flight, like a tray table latch. Even though the plane is not on the ground, there are certain actions that must be done and requirements that must be met. Some of those requirements include the use of placards to mark something as inoperable, do not use, do not sit, etc.
While it is best to have properly made placards that have been laminated, regulations do not prohibit the use of a piece of paper or paper towel and use of tape to mark something as unusable. The aircraft minimum equipment list does interface with flight attendant operations, and flight attendant manuals should contain information on how to address equipment or other features of the aircraft that break during flight so they know what to do should it occur.