Depending on the width of your browser 1px gaps can occur between input-group-addon and the adjacent input (order doesn't matter).

This doesn't occur with input-group-btn due to the table-cell element just being a wrapper and a -1px margin correction is made.

I would like to propose the same idea of just using input-group-addon as a wrapper and applying the -1px margin correction to an inner element.

You can reproduce the issue easily in IE11 with the bootstrap site by going to http://getbootstrap.com/components/#input-groups-sizing inspecting the form and applying a strong background color like blue then slowly resize your browser horizontally. You'll see the background color "flutter" as the 1px gap happens at alternating pixel widths.

I know this may sound trivial but with IE11 being a modern browser I feel this should be tackled if you want developers like us pushing the framework forward by using it on large 6 Million a month page view client project ;)