re: bread or cake in a jar - warning

It can grow botulism. Botulism grows in the absence of air, as in a sealed jar. All the the ingredients are things unsafe to can. Also, the glass jars are not meant for the oven and can shatter and explode. While this may seem like a cute idea, it is very dangerous.
We tell people if they receive one to throw it away.
Don't even open the jar, as if there is botulism it not only can kill or paralyze if eaten, but it can be absorbed through the skin, especially if you have a cut or something. Botulism can grow to deadly amounts within 2 or 3 days in a jar.