"The pair was ..." or "the pair were ..."

Solution 1:

It's really just a matter of style.

Here's an NGram showing that both forms occur about as often. More recent prescriptive grammarians tend to say that a pair must be singular, but that's not going to stop half the English-speaking world from continuing with what was originally the much more common pluralised usage.

The most sensible way to approach this one is to assume both forms are valid. If the usage primarily deals with the pair as a unit, go for the singular. I don't find it easy to visualise a pair drinking together as anything other than two actual people, so in OP's example I would use the plural without hesitation.

By way of support for that last sentence, there aren't enough written instances in Google Books to compare usage for "pair ... drinking", but compare the 4,990 results for "pair were sitting" against 417 for "pair was sitting". The "guesstimate" figures aren't exactly true - when I page through them, GB admits the plural version has only 320 hits. But the singular really has only 79, which is still more than 4:1 in favour of using the plural if the verb implies each individual member of the pair doing something (particularly, people), as would be the case with, say, drinking.

Solution 2:

Important message: I've just edited this question, so it now advocates a (hopefully) much more correct view. It has 4 downvotes from my saying that pair was is the only correct way to say it (which I did believe 9 months ago). I am sincerely regretful for having misled the OP (and anyone else to whom this applies) with not much chance of his seeing my correction.

From a prescriptive point of view, since pair is a singular noun, referring to two things (while pairs is the plural form of that noun), pair was is the correct way to say it:

Hence:

A pair was...

Two pairs were...

The article is therefore grammatically correct.

However, since the world is not composed of pedants, your proposition (the pair were...) is no less correct. Due to a grammatical phenomenon called synesis, pair and other collective nouns can be treated as plural, taking a plural verb:

Synesis is a traditional grammatical/rhetorical term derived from Greek σύνεσις (originally meaning "unification, meeting, sense, conscience, insight, realization, mind, reason"). A constructio kata synesin (or constructio ad sensum in Latin) means a grammatical construction in which a word takes the gender or number not of the word with which it should regularly agree, but of some other word implied in that word. It is effectively an agreement of words with the sense, instead of the morphosyntactic form.

Example:

• If the band are popular, they will play next month.

Here, the plural pronoun they co-refers with the singular noun band. One can think of the antecedent of they as an implied plural noun such as musicians.

I won't quote more, since I included the link, but it's well worth looking at.

The moral of this story is that either way is grammatically correct, and the difference is a matter of style.