Using a vertical mouse for months has seriously fucked up my hand anatomy.
Like, bending my fingers of my right hand down, pressing onto the palm, hurts so bad that I can barely repeat it three times (and I am generally not itsy about pain, especially self-inflicted). My left hand is fine.
And I only found this out after about a year of occupational therapy and three months of self therapy and research. It's not obvious (the therapist didn't see anything), and it's probably that there's a muscle band which grew significantly AND that my thumb slowly shifted into a new place.
This means that I have to get kidteen's a new mouse, because she also uses my vertical mouse with no alternative and I can't have her suffer through this pain.