When there's a mostly-but-not-always correlation between two signals, it can be dangerous.

An example from holding your breath.

The way our bodies decide how badly we should take a breath is the density of CO2 in our blood.

But the actual thing that determines if we need to breathe is the inverse density of O2 in our lungs.

In the vast majority of cases, these two signals correlate very well.

But it's possible for them to diverge!

For example, if you hyperventilate before going underwater and holding your breath (to get more air in your lungs) you expel CO2 better than you take in O2.

This can lead to a situation where your body doesn't realize you need to breathe and you asphyxiate underwater.

Another, less morbid example: "I don't have an easy way to grab onto this idea, therefore it's abstract"