When you lift up an object that was lighter than you expected it to be, it is pretty striking. Not only do you have slight balance issues during the lift (toppling over, in the worst case scenario), but your first thought will likely have been "wow, that was really light". Of course, you will probably have lifted less heavy items in your life before, but what makes the experience really stand out is how it compared to your expectations. You expected it to be heavier than it was, ergo it was lighter than you expected.
These relative judgements of weight can create interesting illusions. In the classic 'Size Weight Illusion', the smaller of 2 equally weighted boxes feels heavier than a larger counterpart. The lifter expected the smaller object to weigh less than it actually did - it was heavier than expected, and thus having its apparent weight boosted. These expectation effects can be extended in many different directions. Similar illusions can be induced with materials, color, temperature, slipperiness, and even semantic cues ("heavy" vs "light" written on different boxes). These expectation-based effects on perception are relatively stable - people still experience illusions of the same magnitude regardless of whether the stimuli are new (and they are lifting them poorly), or whether they are well practiced at lifting them. Even with years of practice, individuals continue to experience the illusion - on some level they are still fooled into expecting the small box to weigh less than the big box, in spite of any evidence that they may have to the contrary. The role that our expectations play our perception of the world has received fairly little study in comparison to, for example, the way that our action can affect perception. A large part of my researh programme is to determine how different stimulus properties (size, material, multimodal combinations that are either consistent or in opposition to one another) trigger our expectations to influence our perception of the world in this context.