```javascript
mutateElementToAffectOffsetTop(someElement);

// (1) We don't know the value of someElement.offsetTop here because we compute it lazily.

var y = (new DOMTransaction(function () {
    // (2) We need to remember the "old value" of someElement.offsetTop here.
    mutateElementToAffectOffsetTop(someElement);
    return someElement.offsetTop;
})).execute();
```

If `DOMTransaction` were to preserve the old value, then `y` needs to be identically equal to the value of `someElement.offsetTop` at (1).  However, since nobody had asked for the value in (1), we're forced to pre-compute the value at (2).  It's worse than that.  `mutateElementToAffectOffsetTop` might be using the old value of other computed CSS values in which case we have to pre-compute all the property values that could be queried by the script at (2).