The gemara in Elu Metzios (Bava Metzia 30b) says:
אמר מר (אשר) ילכו - זה ביקור חולים. היינו גמילות חסדים! לא נצרכה אלא לבן גילו ,דאמר מר בן גילו נוטל אחד מששים בחליו ,ואפי' הכי מבעי ליה למיזל לגביה
My chavrusa and I found that interesting because until we read this gemara, we had thought that the drasha on visiting the sick was that you take away 1/60 of the sick person's illness. Turns out that this is not only a double-edged sword (i.e. the 1/60 illness doesn't just go away, but sticks to the visitor instead!) but also this only applies to someone of the same mazal ("ben gilo") as the choleh!
So we checked out the halacha in the Rambam. Here's what he has to say (Hilchos Avel 14:6):
וכל המבקר את החולה, כאילו נטל חלק מחולייו, והקל מעליו
Hello? What happened to "ben gilo"? Is this Rambam's rationalist way of seeing things, that he summarily changed the pshat from the gemara? The Shulchan Aruch likewise rules like the Rambam, that this healing effect of bikkur cholim applies to everyone. And I haven't found anyone (though my research has been far from exhaustive) who disagrees, or mentions a word about "ben gilo" in the halacha!
So how did this concept from the gemara get struck from the record when it came to halacha?