Many things happen to the fasting person that he does not intend, such as wounds or nosebleeds or vomiting or water going into his throat or gasoline fumes going into his throat, all against his will. Thus, all of these matters do not corrupt the fast. The is due to the Prophets statement: Whoever is overcome by vomiting, he does not have to make up for the day of fasting, and whoever makes himself vomit, he must make up for the day of fasting.
1. Abu Dawud no. 2380. At-Tirmithi no. 720 and Ibn Majah no. 1676.