On this particular page:
http://getbootstrap.com/components/#input-groups-buttons
If you modify the Go! button
to a label (using Chrome inspector), you will observe that the Go! button is no longer positioned on top of the input field:
https://i.sstatic.net/rqnto.png
(Instead, the borders are now next to each other rather than stacked.)
What might be causing this and how can you adjust the elements to stack on top of one another?