Will anything happen if you left a co2 cartridge in a bb gun?

It is generally not a good idea. If the gun is left under pressure it will eventually leak out. Some say it is okay because the new seals are much better than the old ones. But leaving the C02 in the gun makes it potentially dangerous, if someone else picks it up not knowing that it is capable of firing it could injure someone. I own over 30 different air pistols and never leave a C02 cylinder in any of them after shooting. I accidentally left a discharged one in one of my pistols once and a few months later went to use it again and found that the cylinder had gotten stick in the chamber. I had to dissemble the pistol to get the cylinder loose. It was stuck against the seal.