Looking back, I’ve realized how much of what (little) halacha I know was learned because of an offhand remark. I hear some random din, and I look it up to find the context and I end up learning a couple halachos of RaMBaM or seifim of Shulchan Arukh with commentaries. So I’ll start with this one:
On Rashi tefillin, the end of the thread is placed in front of the first charitz (on the shel Rosh), on Rabbeinu TaM the sofrim place it at the second one.