A board does not magically work because the schematic is right. Many subtle effects bite you between schematic and bench.
Power. Decouple every IC's VCC pin with a 100 nF ceramic close to the pin, plus a 10 uF bulk near the regulator. This is non-negotiable. Without it, fast switching currents pull VCC down through inductive trace impedance, and the chip glitches in ways that look like "random" software bugs.
Reset. A clean reset is a high-going edge with no glitches. RC reset circuits are OK for hobby boards; production designs use a dedicated supervisor (TPS3700, MCP120) with a brown-out detector that holds reset asserted until VCC is high enough.
Clock. A crystal oscillator is the gold standard for accuracy: 10-50 ppm. Internal RC oscillators are 1-3 % accurate, fine for blinking LEDs but useless for USB or precise UART. Crystals need two small load capacitors (typically 18-22 pF) and clean layout because they are picoampere-scale signals.
Decoupling and grounding. Star ground for analog. Separate analog and digital ground planes joined at a single point near the ADC. Keep return currents short.
EMI/EMC. Switching currents radiate. Slow your edges (output drive strength), filter cables (ferrite beads), shield critical sections (cans). For consumer products, EU CE / FCC / IC certification require specific emission limits; many designs end up adding ferrite beads and capacitors at the cable exit.
ESD. Anything a human can touch needs ESD protection: TVS diodes on USB lines, on connectors, on antenna ports. A 8 kV human-body discharge can punch through silicon if the path is not diverted to ground.
These are not "extras." A flawless schematic with sloppy power or ESD will fail in the field.