Galen was never able to perform human dissections. The law of his time didn't permit it. So instead, he would study other species (e.g. monkeys) and guess at what must be going on inside humans. If he was lucky, he would encounter a human corpse out and about and get to stare at it a bit, but that was it. Consequently, there were some errors in his neuroanatomy, etc.
Andreas Vesalius (1514 –1564) was not outlawed from performing human dissections. When he came across some of the errors in Galen's anatomy, he noted them and eventually published his report in his book, De humani corporis fabrica. At the time that the Fabrica was published, Galen's word was gospel. So when Vesalius' mentor and devoted Galenist, Jacobus Sylvius, found out that Vesalius sought to correct Galen's observations, it didn't sit well.
Of Vesalius, Sylvius wrote, "Honest reader, I urge you to pay no attention to a certain ridiculous madman, one utterly lacking in talent who curses and inveighs against his teachers." He then went on to write a book entitled A Refutation of the Slanders of a Madman Against the Anatomy of Hippocrates and Galen, wherein he continued, "Let no one give heed to that very ignorant and arrogant man who through his ignorance, ingratitude, impudence, and impiety denies everything his deranged or feeble vision cannot locate." He then basically tried to call the cops on Vesalius to "punish severely, as he deserves, this monster born and bred in his own house, this worst example of ignorance, ingratitude, arrogance, and impiety, to suppress him so that he may not poison the rest of Europe with his pestilent breath." Nice.
*All quotations are from Stanley Finger's The Minds Behind the Brain (2000).