Pliers are more likely to slip off a nut or bolt when:

Using pliers as a wrench is likely to lead to slippage on a nut or bolt because pliers are not designed for that purpose. Pliers typically have a flat gripping surface that does not provide the same level of contact with the workpiece as wrench jaws do. Wrenches are designed specifically to fit around a nut or bolt securely, providing a large contact area that minimizes the chance of slipping. When pliers are used improperly in this way, they may not grip the fastener adequately, leading to a lack of control and potential slippage during use.

Other factors, like inadequate force, lack of lubrication, or failure to inspect tools, can contribute to problems as well, but they are not the primary reasons for the slipping in this case. Proper use of tools is paramount for maintaining safety and effectiveness in any task involving mechanical fastening.