I have noticed that randomly, there is a touch sensitivity issue that happens to the Ultra. Sometimes i find my self tapping really harder than usually required for my touch input to be recognized. A simple tap that usually works gets ignored. This issue seems to go away if i restart the phone.
Now the weird thing is that this is my 3rd Xperia Ultra. The ones i had before had this issue as well.
I am not sure if its a software issue or hardware. its really weird though.