1. You want to setup a connection between the phone and a server, and you need to control both the input and the output. As a result, you use URL.openConnection(), with setDoInput() and setDoOutput() set to true:
URL url = new URL("https://blahblahblah.com");
URLConnection conn = url.openConnection();
At some point you use both conn.getOutputStream() to write to the stream, then conn.getInputStream() to get the response.
2. You're doing an HTTPS connection. Some people report this happening on normal HTTP, but I've only seen it happen on HTTPS.
3. The first request goes through fine and dandy.
4. The second time you try to make the request, the connection doesn't send any data out and doesn't receive any data; it looks like it happens instantly. If you cast to an HttpURLConnection, conn.getResponseCode() returns -1 instead of anything meaningful.
In other words, every other request, the request fails outright. This is a noted bug in Android, but it isn't fixed yet in any released versions. Even when it's fixed, you'll still have to deal with this on older versions of Android.
There are a few workarounds. The first is to simply not use URLConnection; if you can find some way around it, avoid it. The second is to repeatedly make the same request until it works; it's a little too much of a hack for my tastes, but it'll work.
Then there's the third workaround, which I do not claim to understand why it fixes the issue but it does. Just set this setting when your application begins:
Unfortunately this has some drawbacks (keep-alive is a good thing normally), but in comparison to mysteriously failed requests I'll ditch it.