In Thud there was/is a mistake in the print version. The Kindle version seems to have sorted it out.
‘Oh, Sam . . .’ murmured Sybil, the iron mask falling for a moment.
Pratchett, Terry. Thud!: (Discworld Novel 34) (Discworld series) (p. 268). Transworld. Kindle Edition.
The print version mentioned Sybil's "Iron Mast", which made no sense, but I have still had discussions with people that insisted it was "Mast".