The verb mori ("to die") has the unusual past participle mortuus ("dead"). The stem of the participle is mortu-, the only example of a past participle stem ending in a vowel I can think of. (If my memory fails, please let me know!) I would instead expect mortus or perhaps moritus.
The future participle is moriturus. This would give a little discrepancy between mortus (from which one might expect morturus) and the future participle, but this is not unique. While usually the future participle comes directly from the past one by adding -ur-, there are examples of similar behaviour, like ortus and oriturus. See this question for discrepancies between past and future participles.
I never understood why mortuus has two Us instead of being simply mortus. Can anyone explain how this form came about? Was this participle always like this before the classical times? Are there other similar past participles in Latin?
There is a separate question about the pronunciation of mortuus, and all evidence points towards it being trisyllabic. Both Us are therefore vowels.