Can an uncontrollable fusion reaction occur?

Under terrestrial conditions, a spontaneous fusion reaction can never occur. Keeping the fusing fuel in a state where fusion can occur requires constant outside intervention. In the case of magnetic confinement, a proper magnetic fields to hold the plasma in place and heat the plasma to keep it at a sufficiently high temperatures are essential. In addition to these assumptions, a lot of other parameters need to be taken care of and as soon as the conditions are not ideal, the fusion reaction stops and cannot continue. Moreover, less than one gram of fuel is involved in the reaction at one time. So even if all the available fuel were to undergo a fusion reaction at once somehow, the energy released would not be large enough to do any significant damage. In reality, however, scientists are working very hard on a fusion that would burn at least a little bit on its own. They are trying to achieve a state called ignition, where the fusion reactions will produce enough heat that there is no need to heat the plasma further. However, controlled fusion on Earth cannot be done without maintaining magnetic fields, removal of impurities, fuel injection or instability management.