The pasta option in coach is always vegetarian, but if you're in the last 5-10 rows it may not be an option for you. Just kindly ask the crew if it's possible to save one for you as you forgot to order the special meal you needed. The purser in the aft galley is best person to ask as they are the most informed as to the quantities of each meal catered and if there are possibly special meals that were ordered and the passenger is not on board. Politeness goes a long way!