.input-group-addon:last-child

[ahmadramadan]

.input-group-addon:last-child
to fix the black left border we have to add this css :
border-color: #ccc !important;
inside the selector : .input-group-addon:last-child

thanks :)

[sacredseven]

Since i'm not interested in writing my full name on the Contributing page, I just paste the solution here.

The following CSS rule

.input-group-addon:last-child {
    border-left-width:1px;
    border-left-style:solid;
    border-right: 0 none;
}

should be replaced by

.input-group-addon:last-child {
    border-left: 1px #CCC solid;
    border-right: 0 none;
}

.has-error .input-group-addon:last-child {
    border-left-color: #A94442;
}