The dark grey plastic piping in this picture has been known to leak particularly at its fittings/connectors. It is called Polybutelene, PB, or Poly-B piping.

The connectors used to join two pieces of pipe together are typically acetal/grey plastic or copper. These will be framed on either side by a snap ring, in this case copper but sometimes aluminum.

The acetal fittings are more prone to failure than their copper counterparts.