A common mistake in both writing and speech is the misuse of the "past perfect"—or more generally speaking, the inclusion of have or had when they aren't required (along with the other way around). Here, by way of a word-nerdy relative I visited over the holidays, is a wonderful example of the correct use of this tense, where despite the insane repetition of the word had, the sentence itself remains fully grammatical (albeit lacking punctuation and "emphasis"): James while John had had had had had had had had had had had a better effect on the teacher. You just might need a palate cleanser after that linguistic mouthful, so allow me to recommend the A. A. Milne poem "Disobedience." Have fun with this one (if this is the kind of thing you tend to have fun with) and remember this: If you go down to the end of Grammartown, you had better go down with me! There were 27 cases of Repitition* in OhioLINK, and 305 in WorldCat.
(James John Elementary School cornerstone, Portland, Oregon, 1929, from Wikimedia Commons.)