I've spotted another mistake in The CELTA Course Trainee Book by Scott Thornbury and Peter Watkins (see my blog for 4th July 2010). On page 155 the authors discuss how vowel sounds are produced in English and then provide their own definition of a 'diphthong': "a glide from one vowel to another".
Now, this definition is not complete. If it were so, then the words Lyons - the place in France - (ˈliːɒ̃) and neon (ˈniːɒn) would both contain a diphthong - but they DO NOT because they are characterised by two separate syllables.A diphthong is then a movement (or glide) from one vowel to another within a single syllable.
The same mistake can also be found on page 13 of The TKT Course (2005, CUP) by Mary Spratt, Alan Pulverness, and Melanie Williams.