Recently a semi submersible inadvertently unlatched the subsea wellhead connector on a recently perforated well. Upon the wellhead connector unlatching the differential pressure between the weighted completion fluid in the riser and the normal seawater pressure caused the stack to separate from the wellhead and the seal to be lost. The fluid level in the riser equalized to the seawater hydrostatic. The subsequent loss of hydrostatic pressure allowed an influx of crude oil into the well bore. Quick action in getting the connector re-latched and the seal reestablished averted a catastrophic well control and environmental situation.