footnotes.

These had their origin in two sources: the desire to show that one was
honest and to prove it by a reference; the desire to elucidate some
point which it was not easy to elucidate in the text itself without
making the sentence too elaborate and clumsy. Either use may be seen at
its best in Gibbon. With the last generation they have served mainly,
and sometimes merely, for ritual adornment and terror, not to make
clearer or more honest, but to deceive. Thus Taine in his monstrously
false history of the Revolution revels in footnotes; you have but to
examine a batch of them with care to turn them completely against his
own conclusions--they are only put there as a sort of spiked paling to
warn off trespassers. Or, again, M. Thibaut, who writes under the name
of "Anatole France," gives footnotes by the score in his romance of Joan
of Arc, apparently not even caring to examine whether they so much as
refer to his text, let alone support it. They seem to have been done by
contract.

Another ailment in this department is the negative one, whereby an
historian will leave out some aspect which to him, cramped in a study,
seems unimportant, but which any plain man moving in the world would
have told him to be the essential aspect of the whole matter. For
instance, when Napoleon left Madrid on his forced march to intercept Sir
John Moore before that general should have reached Benevente, he thought
Moore was at Valladolid, when as a fact he was at Sahagun. In Mr. Oman's
history of the Peninsular War the error is put thus: "Napoleon had not
the comparatively easy task of cutting the road between Valladolid and
Astorga, but the much harder one of intercepting that between Sahagun
and Astorga."