wire d = 1'bx; // say from previous block. Will be 1 or 0 in hardware

if (d == 1'b) // false in simulation. May be true of false in hardware

wire a;
wire b;
reg q;
    
always @(a) // b missing from sensativity list
 q = a & b; // In simulation q will change only when a changes

In hardware, q will change whenever a or b changes.