"Video won't play" turned out to be four different bugs

Our app ships long video lessons. Twenty to forty minutes each, watched mostly on hostel wifi and mobile data with two bars. Streaming video looks like a solved problem when you build the happy path. You wire up ExoPlayer, point it at a stream, it plays. Then real students open it on devices you have never held, and your bug tracker starts filling up with one line: "video not playing." That line turned out to be four unrelated problems. Here's what I actually learned. ## Buffering is the bug you feel before you see an error Nobody waits out a bad connection during a 35-minute lecture. On a 15-second reel a stall is annoying. In a lecture, a stall every two minutes means the student closes the app and studies from a PDF instead. So the real question was never "does it play." It was "what happens when the network stops cooperating for four seconds." Three situations broke us repeatedly: Wifi to mobile data handover mid-lesson Bandwidth that is technically connected but useless Short total drops, three to ten seconds, that should not end playback ExoPlayer gives you everything you need here. Adaptive bitrate, a configurable LoadControl , player listeners. The catch is that the defaults are tuned for general media, not for a 40-minute lecture on a bad link. What moved the numbers for us: Bigger buffers, deliberately. DefaultLoadControl.Builder().setBufferDurationsMs(...) takes a min buffer, a max buffer, the buffer needed to start playback, and the buffer needed to resume after a rebuffer. That last parameter is the interesting one. Raising it costs you a little extra time on resume and buys you far fewer repeat stalls, because the player stops trying to restart on a nearly empty buffer. Telling the user the truth. Our first version showed a spinner and nothing else. A spinner with no context reads as "the app is frozen," so students force-closed and reopened, which threw away the buffer and made everything worse. Saying "reconnecting" instead of spinning silently cut t