This morning, a colleague sent me this message:
As a data freak, I thought you might be interested in this alert sent to me by Nature.
So, who is the data freak? The initial clause ('as a data freak') is a non-finite subjectless clause, and according to the normative rules of English, the subject of the main clause must be its assumed subject – so my colleague is the data freak! Though he clearly intended it to refer to me. It's a bit like the sentence:
While walking to school, the birds were singing.
In this sentence, 'while walking to school' is similarly a subjectless non-finite clause, so its subject must be the subject of the main clause, 'the birds' – i.e. the birds were walking to school!
But these sentences are rather common, and nobody seems to misunderstand them. In fact, only pedants notice there is anything wrong with them. Or we could alternatively say there is actually nothing wrong with them, and the normative grammar has got it wrong. If the grammar taught by teachers tries to prevent us from using language as we all use it, then that grammar must be wrong.