Spotify. On OSX it will occasionally (every 2 or 3 days) use 100% of my cpu (pretty powerful MacBook air). Thosands of people have complained about the issue for years, yet it still isn't fixdd. I'd like to think a company with the size of Spotify could fix an issue causing so many computers to heat up and waste resources. The desktop app also loves to say I'm offline when I'm not.

Additionally, the UI of their iPad app is so odd, with these sliding panels.