For example,

var xhr = new XMLHttpRequest();
xhr.open("GET", "/foo");
xhr.onloadend = function() { blob.close(); console.log("XHR finished"); }
xhr.onload = function() { finishWorking(); }
xhr.onerror = function() { blob.close(); }
xhr.send(blob);

would result in onloadend throwing and not doing anything after blob.close() on the onerror path, since onerror already closed the blob, even though it's in exactly the state the developer wanted--the blob is closed.

(The above isn't the best way to do this, of course--you should just call blob.close() immediately after xhr.send--it's just an example of something harmless being turned into a bug.)

Blob.close() no longer throws when invoked more than once:

http://dev.w3.org/2006/webapi/FileAPI/#close-method

Can you remove "do nothing, since context object is already closed, and "?

Done: http://dev.w3.org/2006/webapi/FileAPI/#dfn-close