Bret Devereaux

Avoid Gibbon on this point. Our understanding of the Roman army has meaningfully improved and changed substantially since 1789.

(In practice, avoid Gibbon on essentially all points. Decline and Fall was important in terms of the development of the field of Roman history, but isn't really current or accurate on much of any of its main arguments.)

What's a specific example of something relevant I'm likely to be mistaken about by reading Gibbon?

Oh boy. Gibbon's argument about Christianity leading to the fall of the empire is a mess and not taken seriously by historians. His moralizing discussion of civic virtue is likewise the result of an overly credulous reading of the sources.

Gibbon seems blissfully unaware that the Romans had been complaining about the decline of their civic morals since the days of Cato the Elder during the Middle Republic some 650 years before the empire fell and instead takes them all at their word.

His understanding of the late Roman army and the place of the Foederati in it is, as you'd expect, centuries out of date. Archaeology has done a lot to change our understanding of some of these institutions and their changes, but by virtue of writing before...

...the entire discipline of archaeology existed, Gibbon is robbed of the ability to take advantage of it. That also robs him of the ability to make any sense of the economic decline of the empire because, relying on the literary sources, he doesn't know when it happened...