Breadboards

Great for testing concepts and principles

Can get stale when the boad begins to degrade

Wires are friction fit and the sockets and wiring will give out over time.

DIY boards

Wires can still wear down

Parts are sold in bulk for industrial production at small scales; designed more to be built by assembly machines than hand anymore.

Printing your own screens and etching boards: requires volatile chemicals and handling

Cutting the circuit board with a CNC mill: requires a machine to cut traces.

Designing for production

Assuming the first draft of a circuit board will be successful

Test points

Forgetting to include test points for instruments during design

Diagnostic LEDs

Including LEDs to indicate some feature (power, battery) is working properly

Crowding components

Placing all modules as close together as possible.

Makes it more difficult to check and build the prototype boards.

Silkscreen

Should be at least 2mm tall

Silk screens should remain readable after components are populated

Isolation Jumpers

"0 ohm resistors"

Allows disconnecting parts of a circuit to test the component without requiring desoldering

Use all pins on the processors

Even if you do not use a pin--route it to an empty pad somewhere. You can use the pad to make test connections and bodges on the prototype board if needed.

I2C address conflicts

Add jumpers to prototype boards to change I2C addresses if needed.

Modular boards

Break up boards in to sections.

"Mouse bites" are drill holes that make it possible to snap a PCB apart where needed

Ki Kit software mentioned to panelize and split up the boards.

UART mixups

Ensure transmit and receive pins go to the correct place

Add jumpers in case transmit and receive lines have been crossed

Checking for labelled parts

Some manufacturers have stopped labelling passive components. Make sure yours are labelled.

Mismatched footprints

Using foot prints in the EDA software which do not match the chips you are using.

Double check the footprints match the chipset; different vendor chips may work the same but have different dimensions and pinouts.

Laying out without checking parts

Using parts that you do not have and cannot actually obtain.