First, read this extract from The Economist of 4 April 2009, p. 11:
Traders and fund managers got huge rewards for speculating with other people's money, but when they failed the parent company, the client and ultimately the taxpayer had to pay the bill.Now ask yourself this: who had to pay the bill? Was it the client and the taxpayer? Or was it the parent company, the client and the taxpayer?
In fact, the intended meaning is the latter: the parent company was supposed to be included as one of the parties that had to pick up the bill. And this meaning would have been expressed so much more clearly if there had been a comma in the final clause of the extract:
... when they failed, the parent company, the client and ultimately the taxpayer had to pay the bill.The problem here is that fail can be either an intransitive verb (so it has no object) or a transitive verb (with an object). In the intended meaning of this extract, fail is an intransitive verb: when the companies failed, various parties had to do something about it. But when there is no comma after failed, our first instinct is to treat the noun following it as its object.
When we write, we should always be sensitive to possible ambiguity. Sometimes, adding punctuation can make things clearer. In other cases, it is best to re-word what we have written to ensure the intended meaning emerges properly.
One other thing about this example: at times, a bit of grammatical terminology (concerning transitive/intransitive verbs) can help one to see and also explain ambiguity.
