An underscore crash Internet Explorer 6

Until some minutes ago my About page contained an empty div with id attribute set to left<strong>_</strong>column.

It was useless – so now is gone – but the remarkable thing is that the underscore character crashed Internet Explorer 6.

I don’t know anything about a bug of this type: many other elements of my sites have names containing “_”. How is it possible that a simple underscore crash IE? And why it has not happened before?