This is just pseudo-code, but it (roughly) crashes PHP:

Function foo()
{	while( 1 )
	{	switch( "b" )
		{	case "a":
				break;
			case "b":
				if( 1 )
				{	switch( "foo" )
					{	case "A":
						case "B"
						break;
						default:
						return 0;
					}
				}
		}
	}
}

The key here is the code:

``
case "A":
case "B"
	break;
''

Placing the ``:'' after the B case works normally, but without it PHP crashes (Document contains no data).

As an aside, statements like this:
if( something )
}

*really* confuse the parser.  Instead of getting an error here, I get undefined function errors, which is really misleading.

regards,
Christopher