A programmer reports this problem when using XML: I forgot to put the end tag on one of my XML tags, and everything quit working. Took me forever to figure it out, and I had to use IE as a debugger....

A programmer reports this problem when using XML: “I forgot to put the end tag on one of my XML tags, and everything quit working. Took me forever to figure it out, and I had to use IE as a debugger.” What kind of error is it? What rules were applied, and which should have been applied earlier?