I worked on a Windows Mobile (on the huge ruggedized devices) they were really slow, but the people who used them knew the screens so well, that they wouldn’t wait on the screen to refresh to navigate the screens and start typing. They would just take advantage of the keyboard buffering. I mistakenly changed the order of one field on the screen and it threw off all of the data entry. We could not reproduce the “bug” but we knew we shouldn’t be getting that many mistakes from the same people who were doing everything correctly. We actually sent the QA guy out in the field and he discovered the issue within an hour - the users weren’t looking at the screens, they were doing everything from muscle memory.