Made a timer app with full-screen start/stop touch area for Wim Hof breathing exercises (hitting the small circle on the built-in app was lowering the precision of my breath-hold time measurements). Got a cold after a few weeks and never really got back into it.

A few months later my iPhone 6 died. Got the suspicion that maybe my app was somehow always running in the background, wearing it out. Didn't let the Apple store guys in on my suspicion when I went to replace it. iOS was probably already killing background apps by then but I'm still not sure.