Weighing reaction mixture
Think about the reaction:
$$\ce{NaCHO3(s) + CH3COOH(l) \to CO2(g) + H2O(g) + Na+(aq) + CH3COO- (aq)}$$
The resulting $\ce{CO2}$ will make the bag expand, therefore increasing the volume and the subsequent buoyant force. This force opposes the force of gravity, making it seem like the mass of the matter inside the bag has decreased.