Who's that on the motorcycle?

"Dangling modifiers", so-called, are created when a subordinate clause (usually participial) whose subject is not expressed has a different subject than the main clause, yet is placed so that it seems to have the same one. They are enormously easy to write. Most of the time we don't even notice them, reading the author's intended meaning without trouble because the pragmatics of the text allow no other sensible reading. In fact, we tend to notice them only when they introduce an ambiguity that cannot be resolved, so that we puzzle over them, or when they in fact create a misreading that we stumble over later in the text, or when they're so incongruous that they're funny. (The guys at Language Log write about them at times; here's a list of such posts.)

Well, today Clemmie Moore of the UK's Daily Mail started off a story with a doozy:

Riding a vintage motorcycle and sporting a raffish moustache, you could be forgiven for thinking he was filming his latest movie.

I never sport a mustache, raffish or not, and have only even been on a motorcycle twice in my life - and it wasn't vintage.

It's not the structure per se - the "subjectless gerund-participle + main clause" - that's a perfectly valid structure in English. In fact, this sentence is perfectly grammatical. It just doesn't mean what she wanted it to mean. But she could have started with "Seeing him riding..." and all would have been copacetic.

The lesson here isn't to avoid subject-less participial (or participle-gerund if you like) clauses, it's (as always) to be careful with your modifiers.

ps - The story's next paragraph (I love one-sentence-paragraph news stories, don't you?) is:

But this was Orlando Bloom enjoying some time away from the set.

Oh.