docs

docs/api.md

@@ -513,6 +513,21 @@ switch (this.accepts('json', 'html', 'text')) {
 
   Request socket object.
 
+## Error handling
+
+  By default outputs all errors to stderr unless __NODE_ENV__ is "test". To perform custom error-handling logic such as centralized logging you
+  can add an "error" event listener:
+
+```js
+app.on('error', function(err){
+  log.error('server error', err);
+});
+```
+
+  When an error occurs _and_ it is still possible to respond to the client, aka no data has been written to the socket, Koa will respond
+  appropriately with a 500 "Internal Server Error". In either case
+  an app-level "error" is emitted for logging purposes. 
+
 ## Notes
 
 ### HEAD support