monkey wrench [munhg-kee-rench]
1. a wrench with an adjustable jaw permitting it to grasp nuts or the like of different sizes.
2. something that interferes with functioning.
3. an obstacle
EX. In J. K. Rowling's best-selling Harry Potter series they introduced a time travel device in the the third book, Harry Potter and the Prisoner of Azkaban. Despite the fact that this object has the quality to allow its user to go back as far as they can turn back the device, one hour per turn, they gave it to Hermione Granger so she could take multiple classes at once. They very vaguely imply some sort of danger the device can cause but, according to the evidence presented in the book, it is practically fool proof. Therefore, we can only assume that J. K. Rowling allowed it to fade into the background as to not become a monkey wrench in the gears and cogs that move the well-timed machine that is the Harry Potter Universe and timeline.