Often specifically about JLCPCB and KiCAD.

1. If a crystal says 20pF, it probably actually wants caps of 35pF each. C = 2L-5pF, or so. Then pick an available capacitor near that value.
2. JLCPCB’s understanding of 45* rotated components is 90* off of KiCAD’s, in some way. …But only on the pcb bottom? Expect also disagreements about the orientation and position of headers and some ICs.
3. JLCPCB does not like components with commas in the name. OR pipes. Parentheses appear to be ok?
4. Searching JLCPCB for parts:
   • I usually sort by stock descending, in the hopes later orders of the same thing won’t have components out of stock. (Replacing non-trivial components is AWFUL.)
   • If you’re looking for components that are e.g. exceptionally fast or high voltage, try adding a minimum stock and sorting by price descending – since the high-spec parts are usually more expensive.
5. More things as I discover them.