My hovercraft is full of eels

Hah! My personal favourite would be arrays... adding numeric items to them changes their length, while adding non-numeric items doesn't - instead, they become properties of the array object. You can't get at them via the array syntax, but using item() works - unless you are using IE in which case the array syntax works also (and _really_ makes things confusing). Oh, and iterating the array only returns the numeric values, but iterating the items of the array returns the numeric and non-numeric values.

Throw in some test data which, _by coincidence_, is all numbers, and watch the fun begin.

Actually the array thing is interesting because Arrays are actually more like Maps (a.k.a. associative arrays) than the ones we're used to in Java. They just have "special" processing for numeric keys. Then of course there are methods like push and shift which make them more like stacks/queues. Egads!

Um, no... arrays are real arrays. The problem is that _every_ Javascript object is an associative array of properties and functions. And arrays, of course, are Javascript objects, so they are also associative arrays of properties and functions, one such property being an array of values to which certain functions apply. The real bummer is that you can, under IE (which I was writing this Javascript for), use the same syntax for accessing the array as you do for the properties!

The extra special bonus funkiness being, of course, that Javascript doesn't have any types, so IE was inferring the "numericness" from the value, and doing different things with it. Fun stuff... I ended up ditching arrays for what I was trying to do and used a prototype object instead.

Of course, however, once you get past Javascript as a language, you then hit the inconsistent DOM models between different browsers (or are you using Javascript for non-browser work?). But you can't fairly put the DOM problem in the Javascript camp, as tempting as that may be. ;)

Hehehe...well that's what I was getting at: arrays are associative but with some funky behavior to handle integer keys. These are equivalent (thanks to weak-typing) assuming var a = new Array():

a[1] = "hello";
a["1"] = "hello";

In both cases a.length = 2 and a[0] = undefined.

Classes are really just a function--acting as the constructor--which--because it's also an object and all objects are associative arrays--has a list of properties, one of which is "prototype", the contents of which are used when constructing other objects.

It's all kinda weird but somehow I really like it, especially because it's all quite regular--assuming you understand what regular means ;-)