Mutations always occur in nature otherwise we would have no variegated plants like aeae or seedless bananas. I am sure in culture chemicals can be added at some point to induce mutations such as colchicine in the production of tetraploid plants (daylillies and orchids for example). Due to the nature of tissue culture the rate of mutation is probably higher or at least the survival rate of unusual plants (selectively cared for by the grower) since survival of offshoots in nature is not 100%
