ssh: connect to host xxxx port 22: Connection refused

Sometimes, in this complex built world,

(TEDTalks: Thomas Thwaites: How I built a toaster -- from scratch)

you forget about something, you took it for granted for a long while and when something doesn't work for you, you try to find the cause of the error, but cannot find it,

you delve deeper into the problem, and then,

(image from here.)

realise, what an obvious cause it was and you just didn't even think of it...!

Like,

1) in one of my digital circuit experiments, we thought we did everything right, but our circuit board didn't just work. We tried to find the problem for a long while and, D'OH!!! the power supply was not connected!!!

2) you started to use or moved into a house with one of the modernised kitchen equipments, and you wonder why the water on your electric cooker doen't cook!

(Image from here.)

and you realise, you didn't notice there's an energy-saver power-timer you needed to rotate!

(Image from here.)

3) and here's another one.

(Image from here.)

I wanted to SSH connect to my Linux virtual machine, but kept seeing the

ssh: connect to host xxxx port 22: Connection refused

message.

I checked everything else, like firewall settings and virtualisation software settings etc.

And realised, I didn't install SSH server on the virtual machine...

D'OH!!!

(p.s. does anyone have any other good examples of those common cases?)