Well what you are describing...already happen countless times. First time in the 1520s by Ferdinand Magellan and countless times after that

If anyone from the believers in flat earth managed to go for that expedition, he would just find out what everyone already knows, then he would either be ashamed of his own stupidity and pretend he really reached the edge of the world, or say the truth, in which case flat earthers would claim that he is a scammer that participates in the conspiracy xD