The <merror> example from the MathML specification.
<!DOCTYPE math PUBLIC "-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN" "http://www.w3.org/Math/DTD/mathml2/xhtml-math11-f.dtd"> <math xmlns="http://www.w3.org/1998/Math/MathML"> <merror> <mtext>Unrecognized element: <mfraction>arguments were:</mtext> <mrow> <mn>1</mn> <mo>+</mo> <msqrt> <mn>5</mn> </msqrt> </mrow> <mtext>  and  </mtext> <mn>2</mn> </merror> </math>
Author: Robert Miner and Jeff Schaefer, Geometry Technologies