I've had a couple of replacement Calor propane bottles and connected them to the regulator without a thought. I always check that the cone on the regulator and the socket in the bottle are clean and spanner them together but don't test. Never gave it a thought!
This time around, connected the new bottle but thought, though not sure, that there was a slight whiff of gas at the regulator connection. So what do I do? Ha! Also without thinking, put a flame from my gas ignitor near the joint and imediately got a ring of fire!!

Anyway, went back to the marina where I got the bottle and no way could we get the joint to seal. Got another bottle - perfect seal.
Three lessons learnt.
1. Replacement bottles can be defective
2. Always test the regulator joint when replacing a bottle.
3. When testing, use a proper test spray or soap solution.

Cheers!
Pete