A perfect crystal should be enormously strong: to deform it you would have to slide whole atomic planes over one another at once, which needs a shear near the ideal strength, roughly a tenth of the shear modulus. Real metals yield at a thousandth of that.
The resolution, proposed by Taylor, Orowan and Polanyi in 1934, is the dislocation: a single line of misregistry — one extra half-plane of atoms. It glides by breaking just one row of bonds at a time, like a ruck travelling down a carpet, so the crystal deforms at a tiny fraction of the ideal stress. That is why metals are malleable.
To make a metal stronger you do the opposite of what intuition suggests: you add defects. Solute atoms, precipitates and grain boundaries pin the dislocations, and a pinned dislocation cannot glide until the stress is raised. Alloying, work-hardening and grain refinement all come down to this one idea — jam the dislocations.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.