Early in my embedded career I sent a 4-layer board to fab, paid for assembly, and got back boards that browned out under load. $4,000 and six weeks gone. Here's what I got wrong, so you don't have to learn it the expensive way:

1. Ground plane stitching. I split my ground plane to "keep things clean" without stitching vias tying the sections back together at RF-relevant intervals. Return currents took the path of least impedance — straight through my analog front end. Rule of thumb: stitch vias every λ/20 of your highest frequency of concern, always.

2. Decoupling cap placement, not just value. I had the right 100nF/10uF combo on every rail, but placed them after the trace ran under the chip instead of directly at the pin with the shortest possible loop to the ground via. Inductance from trace length matters more than most datasheets tell you.

3. Thermal relief on power vias. I used thermal relief spokes on vias carrying real current (not just for solderability) and created hot spots that soft-failed my regulator under sustained load. High-current vias should be solid-connected, not spoked.

None of this is exotic — it's just the stuff nobody tells you until you've paid for it. I built out a hands-on course (schematic → layout → DFM → bring-up) that walks through real boards, real mistakes, and how to catch them before you hit "submit order." If you're an engineer trying to go from "it works on the breadboard" to "it ships," it might save you a respin or two.