An excerpt from Leo Rosten's "O Kaplan! My Kaplan!":
"Excellent... And now, class, let me take one moment to note that there are certain exceptions-"
Groans instantly greeted the ominous 'exceptions'. The beginners' grade had long ago learned to fear - nay, loathe - the Exception to the Rule. It was the bane of their learning, a snake in the garden of perception. (Mr. Krout, the seniour instructor in the ANSPA, once enlivened a faculty meeting by declaring that 'the very bete noir of English is the skulking multitude of Exceptions to the Rule!' How Mr. Parkhill had admired the way Mr. Krout had put that! He admired it almost as much as he admired Mr. Robinson, the school principal, for responding: 'I heartily agree, Mr. Krout. There are as many exceptions to the rule in English as there were thieves in Baghdad!' Who could forget such a simile? 'But we cannot change the rules of grammar - nor, if I may say so, can we exile all the exceptions... Carry on! That is what we all must do. Carry on!' If there was one quality Mr. Parkhill had inherited from his ancestors, it was the capacity to carry on.)
Why does this Past Perfect appear suddenly and suddenly end? The previous sentence is cast in Simple Past (not "had enlivened"). The next sentence carries on in Simple Past too, in spite of the fact that it tells about the same feeling of admiration.
The sentence stands out from the surrounding parts of the passage by being cast in a "deeper" past. But how can it be "deeper" in the past than the sentence with "once enlivened"?