Are you referring to when you say Calvinists are "reading something into the text" that is not there?
Not necessarily.
We all depend on interpretation and to an extent read into a text. This is a part of understanding, even if it is merely mental pictures that form in our minds.
I am talking about building theory on theory.
Here is one example:
1. Adam's nature changed with the Fall
2. Spiritual death is a separation from God
3. Adam experienced spiritual death
4. At judgment sinners will experience spiritual death
5. Divine justice is retributive justice
6. Sin can be punished without the one who actually sinned bearing that punishment
7. Jesus could bear our sins in the context of God viewing Jesus as if He were guilty
8. God has to exercise punishment on sin
9. God punishes Jesus for our sins instead of punishing us
10. Jesus had to experience this spiritual separation in order for divine justice to be satisfied
11. God had to separate from Jesus for three hours on the cross
Notice that none of those eleven statements are actually in Scripture. Several of those comments are interpretations or theories directly from Scripture. A few are philosophical ideas. But none are actually found in the biblical text. So if you put all of those together you end up with a narrative that, if true, was hidden rather than presented in the actual text.
Here is another example (Origen's view)
1. Because Adam sinned death entered the world.
2. God put enmity between Satan and mankind
3. Christ had victory over sin and the grave
4. We were ransomed by the blood of Christ.
5. God paid this ransom to Satan
All but the interpretation is actual Scripture. While you and I would agree the interpretation is wrong, it is an interpretation of Scripture rather than a systematically derived narrative built on theory.