The CTRL+D find_under_expand command has inconsistent behavior with a selection:
Place the cursor anywhere on the first line and press CTRL+D three times to yield the following three selections:
Clear the selection and select "word" on the first line (via CTRL+, double click, or click and drag).
Attempt to repeat step 2. To select each instance of "word", you end up pressing CTRL+D four times and select the non-word-boundary instances of "word" as well:
I was also looking for a solution to this problem and found a comment on another topic by adzenith that answers this problem.
I know this thread is old but I thought I'd leave the answer here for completeness.