I stumbled upon the following example sentence in a Russian article about English grammar (not articles):
There were a lot of people at a conference.
The use of the indefinite article does not seem correct to me: I suppose, we know what the conference it is since we know how much people were there.
In the comments, I found that it is not only me who noticed that. The authors give the following explanation: there is no enough context to claim that it is a specific conference. When we mention something the first time, we use the indefinite article.
The question is: is the use of the indefinite article justified in this case?