Android when you go one level deeper than the most high-level api. It's easy to record a file with AudioRecord(), but getting the audio stream to handle some analysis yourself is way harder. I'm not the best android programmer and had to read the C code underneath to get to the stuff below the "MediaRecorder".

Oh and Android design standards and best practices... Nobody tells you which stuff is outdated and that you need to use compatibility stuff by default to keep things working on more devices etc... Also no technical description on how to implement Material Design at all and some quirky problems are really common (e.g. negative margin on a button, easy in html/css, but impossible that way in Android).

edit: my new favourite android fuck-up though is having the icons for horizontal and vertical layouts swapped: http://i.imgur.com/xJ1ODI3.png "vertical" = child elements are underneath each other.