Even tire guys get a flat tire on occasion.
I had one last week, and while on the side of the road getting my spare on, I though... hmm the spare tire feels ready to go. I installed it.
After tightening all the lugs, I noticed the spare looking low. It felt as good as a standard tire for inflation pressure - I hooked up my ContiComfortKit, I was at PSI 20. I filled it to the 60 recommended PSI (always fill temp spares to their max).
How the CCK works.
When I was placing my spare back in the trunk, I found it impossible to fit it in fully inflated!
The manufacturer had only allotted for the diameter under-inflated. The moral of the story is always take your spare's pressure, and correct it before you leave!
Keeping an Air Gauge in the car is also a wise idea.