The EventEmitter appears to pass a minimum of two arguments to any bound event handlers, regardless of how many arguments are passed to the emit() function.

possible-node-js-event-emitter-bug.js

// using node v0.2.4
var sys = require('sys'),
	events = require('events');

// create a custom EventEmitter
function CustomEmitter() {
	// call "super" constructor
	events.EventEmitter.call(this);
}
// inherit from EventEmitter
sys.inherits(CustomEmitter, events.EventEmitter);

// create a test instance
var test = new CustomEmitter();

// bind a handler to the 'foo' event
test.on('foo', function() {
	// log the arguments
	console.log(arguments);
});

// run some tests:

test.emit('foo');
// logs 2 args: { '0': undefined, '1': undefined }

test.emit('foo', 'bar');
// logs 2 args: { '0': 'bar', '1': undefined }

test.emit('foo', 'bar', 'baz');
// logs 2 args: { '0': 'bar', '1': 'baz' }

test.emit('foo', 'bar', 'baz', 'hello', 'world');
// logs 4 args: { '0': 'bar', '1': 'baz', '2': 'hello', '3': 'world' }

Update: the source of the unexpected arguments is line 27 of events.js (https://github.com/ry/node/blob/v0.2.4/lib/events.js). This behavior exists all the way up through label v0.3.1 (currently the latest).

How big of a performance hit would you take by potentially removing the 'fast' case?

You could maybe do it this much less elegant way to get the best of both worlds:

if (arguments.length <= 3) {
      // fastish case
    if(arguments.length == 0) {
        handler.call(this);
    } else if(arguments.length == 1) {
        handler.call(this, arguments[1]);
    } else {
        handler.call(this, arguments[1], arguments[2]);
    }
}

I agree. It was suggested in the IRC channel (by webr3) to simply handle the "fast cases" (0-5 arguments or so) with a switch/case, and then do the Array.prototype.slice for higher numbers of arguments. I generally like the idea - think it would make sense to run some actual tests though to see where the hand-off should occur.