What version of async are you using?
2.1.4

Which environment did the issue occur in (Node version/browser version)
Node 6.9.4

What did you do? Please include a minimal reproducable case illustrating issue.

const _ = require('lodash');
const async = require('async');

async.auto({
	'one': function (next) {
		_.defer(function () {
			console.info('Error');
			next('Something bad happened here');
		});
	},
	'filter': function (next) {
		_.delay(function () {
			console.info('Fine');
			next(null, 'All fine here though');
		}, 1000);
	},
	'finally': ['one', 'filter', function (a, next) {
		_.defer(next);
	}]
}, function (err, auto) {
	if (err) {
		console.error(`Caught: ${err}`);
	} else {
		console.info(auto);
	}
});

What did you expect to happen?
Logs error but doesn't raise exception.

What was the actual result?

TypeError: fn is not a function
    at C:\somewhere\node_modules\async\dist\async.js:1594:13
    at arrayEach (C:\somewhere\node_modules\async\dist\async.js:1298:9)
    at taskComplete (C:\somewhere\node_modules\async\dist\async.js:1593:9)
    at C:\somewhere\node_modules\async\dist\async.js:1619:17
    at apply (C:\somewhere\node_modules\async\dist\async.js:21:25)
    at C:\somewhere\node_modules\async\dist\async.js:56:12
    at breakLoop (C:\somewhere\node_modules\async\dist\async.js:840:16)
    ... cut ...

This is because of:

1. listeners variable initially declared as an object, being transformed to array at some point (line 1614) for some reason, which is a bad idea in general.
2. Even if it was an object at that point, it doesn't guarantee anything, without additional hasOwnProperty check. Someone might as well call task hasOwnProperty.