What language bugs have tripped you up?

[Axel Rauschmayer]

What language bugs (as opposed to pure browser bugs, e.g. DOM-related bugs) have tripped you up? What did you rely on that wasn’t supported in older browsers? Two examples that I’ve encountered:

• Older Internet Explorers don’t support the property “name” of functions
• Older Internet Explorers don’t enumerate a property if there is another property with the same name in the prototype chain that is not enumerable.
  https://developer.mozilla.org/en-US/docs/ECMAScript_DontEnum_attribute