According to Charles Spitzer :
63/37 solder solidifies almost instantly.

"almost instantly" isn't "instantly".

Try this experiment: solder a pipe joint while rotating the fittings.
The joint edges won't be shiny, and there's an extremely good chance
it will leak under pressure.

Solder tends to harden via crystalization. If moved during the critical
period, the crystals will literally rotate, and the final solidification will
be filled with voids, and be much weaker.

This is an issue with electronics soldering too.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.