The problem was the notify filters. The program was trying to open a file that was still copying. I removed all of the notify filters except for LastWrite.
private void watch()
{
FileSystemWatcher watcher = new FileSystemWatcher();
watcher.Path = path;
watcher.NotifyFilter = NotifyFilters.LastWrite;
watcher.Filter = "*.*";
watcher.Changed += new FileSystemEventHandler(OnChanged);
watcher.EnableRaisingEvents = true;
}