If a chopped piece is almost falling off the board, gently nudge it back onto the board with your knife or hand to prevent it from falling and creating a mess or potential hazard.
When chopping vegetables or other ingredients, it's common for pieces to shift and move towards the edge of the cutting board. If a piece is about to fall off, the best approach is to act quickly and carefully. First, assess the situation. If the piece is only slightly over the edge, you can use the dull side of your knife to gently push it back onto the board. Alternatively, if your hands are clean, you can use your fingers to nudge it back. The key is to avoid any sudden movements that could cause the piece to fall or, worse, cause you to cut yourself. Once the piece is safely back on the board, you can continue chopping as needed.
To minimize pieces falling off the board in the first place, use a cutting board that is large enough for the task at hand and consider using a damp towel underneath the board to prevent it from slipping. This will provide a more stable and secure surface for chopping.