Bare bones Arduino not working?

ardI built  a bare bones Arduino that when I fired it up, wouldn’t work. Could not connect to it and the blink program that I had put in the chip did not give any output. So I had to start a fault finding exercise in which I checked all the connections, took out the chip to test it in another board, tested the resonator all without effect.
Thinking that maybe there was something wrong with the power supply (after all, I had checked everything else), I hooked up the circuit directly to a 5Volt supply of an other arduino.
That worked, so the problem was in the PSU. yet, I had measured the Voltage and that was a stable 4.95 Volt.
Then a possibility dawned upon me. In my haste to test the circuit, I had not given the 7805 any capacitors yet.
A secondary capacitor of 10uF did the job. When that was soldered in, everything worked like a charm.
So… if you are faced with a similar problem: a self built  ‘Arduino’ that doesn’t work and you really have checked all the connections… make sure u did not forget the cap with the 7805