In the latest season of Game of Thrones Stannis attempts to march onto Winterfell despite the incoming winter. On this march he is attacked by Ramsey Bolton and "20 good men", who sneak into his camp and set fire to his food stores and kill/release his horses.
Now, obviously the show is a dramatization and fiction, but is there any real life examples of guerrila attacks like this being executed to destroy food shelters? Is this just an invention of the show, or were such tactics relatively common? And finally, is it even feasible for a small band of 20 men to sneak into a military camp to cause the sabotage?
Since its fiction it is difficult to draw an exact time period this question relates to, but I would estimate at around 1300-1500 being most accurate.
Raids and small unit actions were integral to medieval warfare, but generally the results were far less dramatic than GoT's depiction. A medieval army would constantly be sending cavalry patrols out to scout the area and forage for supplies. Their reconnaissance was vitally important to the success of any campaign, although they wouldn't be responsible for something as dramatic as destroying an entire army's supplies or releasing all their horses. Their role was much more ordinary: finding out where the hell the enemy's at. Most medieval battles were preceded by days of running skirmishes between mounted patrols as the armies drew closer to each other.
If you wanted to deny food to an enemy army, it would be more common to remove/destroy supplies from villages along the path of their advance than to try to attack the enemy camp directly. Night raids on a camp could and did happen, but they were difficult to pull off successfully. They would also involve a larger force than twenty men, more likely a few hundred or a thousand.