XML encoding problems
Attractive Nuisance contains a link to an interesting slideshow about XML encoding problems, starting from charsets going over to attribute order, whitespaces, entities, double escaping etc.
The slide titled "QNames" just reads "don't even get me started". If everyone understands that XML namespaces are broken, why doesn't anyone really do something about it? The W3C seems to have failed in this issue, both with ambigous URIs, difficult to make out QNames and a really strange way of namespace scoping.