Magnetic field lines are spatially chaotic at a given time when the space Hamiltonian from which they derive is non integrable. As the minimal number of degrees of freedom required for the possible emergence of chaos is three, situations or models involving only two effective space dimensions cannot develop a spatial chaos of magnetic field lines. The question behind this presentation will be: “What is the impact of the spatial chaos of magnetic field lines in hot magnetized plasmas?” Two situations will be addressed: i) in tokamak sawteeth, that can be viewed as a laboratory prototype of magnetic reconnection, evidence will be given that magnetic field lines are spatially chaotic during the fast crash (reconnection) phase. ii) the spatial chaos of magnetic field lines will be shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured in toroidal magnetic devices.