sqwish incorrectly combines pseudo-classes with the previous selector when it is unqualified.
styles.css
.class :first-child {
color: red;
}
styles.min.css
.class:first-child{color:red}
Qualifying it with an asterisk seems to be an effective workaround. On a related thought, but not necessarily related to this bug, it could probably be removed when minifying to save a byte.