Everyone knows you should keep a straight back while lifting yet it amazes me when I see trainers failing to recognize or correct poor technique.
The picture below shows very clearly why maintaining a neutral spine when lifting is important as it shows the loads involved and how just a small degree of variation can shift the load significantly.
Remember, it's not just about the weight being lifted. It's also about how the leverages can change thus applying different loads to different areas in the same lift.
In the example below you can see how a 50kg load can vary between 380 and 630kg of pressure on the disks of the lumbar spine.