• Browser: Chrome version 40.0.2214.91 m
• Bootstrap: 3.3.2
• Expected: 4px border-radius on all four corners of
• Got: 4px border-radius on left; 0px border-radius on right
• JSFiddle: http://jsfiddle.net/mqov56r8/

    <div class="btn-group" role="group" aria-label="...">
        <form class="btn-group">
            <button type="button" class="btn btn-default">Right</button>        
            <input type="hidden" name="_method" value="put" />
        </form>
    </div>

The rule in question seems to be:

    .btn-group>.btn:first-child:not(:last-child):not(.dropdown-toggle) {
        border-top-right-radius: 0;
        border-bottom-right-radius: 0;
    }

Changing the selector to the following fixes this issue, but may cause issues elsewhere:

    .btn-group>.btn:first-child:not(:last-of-type):not(.dropdown-toggle)

Cheers,

Tom