text.sourceText evaluates at comp time, not

Try this:

1) Create a new text layer, and animate the Source Text property.
2) Add an expression to the Source Text property, leaving the default: 

      text.sourceText




3) Slide the layer down the timeline a few seconds.
4) Scrub over the keyframes.
5) Disable the expression, and scrub over the keyframes.

Here's what should happen: the text should behave identically with the
expression enabled as it does with the expression disabled, since the expression
should simply be passing the original value through.

Here's what happens (at least for me): the text animates as if the text had
never been moved on the timeline. Rather than changing at the keyframes, the
text changes where the keyframes would fall if the text layer started at time =
0.

A bug, indeed.