I'm familiar with the rule that if a word ends with a vowel, “s”, or “n”, then the stress is on the second-to-last syllable. Obviously this changes when one of the letters has an accent mark. However, I think I've discovered an exception to the rule, which is the name Luis. The way that this name is pronounced, it seems that there should definitely be an accent mark over the "i".
Do you know of any other exceptions to this rule that don't have accent marks?