What condition or disease causes a mispronunciation of words or trouble with speech?

Could be brain issue: It could suggest a problem in a part of the brain controlling how you control your tongue and lips to pronounce words correctly, the nerves that lead from the brain to those muscles, or a problem in the muscles themselves. It is important to get this evaluated asap to make sure you can catch problems early when they may still be reversible. Good luck.

If new, get seen!: I agree with Dr. Allen. Also, illegal drugs, injuries to the head, some prescription or over-the-counter medications, supplements, & alcohol can all affect speech and coherency, If this is a new problem, it is important to get seen very soon, and even consider ER evaluation if sudden onset (and not due to alcohol). It may turn out to be minor and temporary later, but take no chances now!