buggy2.js

var fs   = require("fs"),
	file = fs.openSync("aa", "a+"),
	bufs = {},
	off  = fs.fstatSync(file).size,
	offs = {};
	
  
Object.keys(offs).forEach(function(o) {
	offs[o] += off;
});

function buf(size) {
	if (!bufs[size]) {
		bufs[size] = new Buffer(size);
	}
	
	return bufs[size];
}

function write(size, callback) {
	var offset = off;
	offs[size] = offset;
	off += size;
	fs.write(file, buf(size), 0, buf(size).length, offset, callback);
}

function reader() {
	console.log(bufs);
	console.log(offs)
	Object.keys(bufs).forEach(function(size) {
		size = +size;
		var offset = offs[size],
			test   = new Buffer(size);
		fs.read(file, test, 0, test.length, offset, function() {
			if (test.toString("base64") != buf(size).toString("base64")) {
				console.log("WTF? " + test.length + ":" + buf(size).length);
				console.log(buf(size).slice(0,4).toString("base64"), test.slice(0, 4).toString("base64"))
			}
		});
	});
}

for (var i = 3900; i < 4000; i++) {
	write(i);
}

setTimeout(reader, 1000);