Very often the verb is separated from its real nominative or subject by several intervening words and in such cases one is liable to make the verb agree with the subject nearest to it. Here are a few examples showing that the leading writers now and then take a tumble into this pitfall:
1."The partition which the two ministers made of the powers of government were singularly happy."—Macaulay.
(Should be was to agree with its subject, partition.)
2."One at least of the qualities which fit it for training ordinary men unfit it for training an extraordinary man."—Bagehot.
(Should be unfits to agree with subject one.)
3."The Tibetans have engaged to exclude from their country those dangerous influences whose appearance were the chief cause of our action."—The Times.
(Should be was to agree with appearance.)
4."An immense amount of confusion and indifference prevail in these days."—Telegraph. (Should be prevails to agree with amount.)