It wasn't so unusual as to be unheard of, but neither was it doctrinal.

Harry Yeide, author of "The Infantry's Armor", briefly addresses this pintle MG issue at 16 seconds into this video. https://youtu.be/wOpU7ENcZvY?t=15

The positioning of the Cal. .50 is a long one, with different parts of the Army coming to different conclusions as to where the thing should be. (I have a report around here somewhere of Tank Destroyer Corps recommending that an MG on one of the new TDs be re-positioned to one place, and Armored Force working with the second prototype wanted it back in the original place).

The advantage to having the MG mounted on the pintle to the rear was that one can get a good angle of elevation for the anti-aircraft role: By standing on the rear, with the gun facing forward, there is no interference with the elevation, and one can still sight along the gun. In the absence of the Luftwaffe, however, it was a bit pointless, so as Harry mentions, some units simply removed the thing entirely. Other units re-positioned the Cal. .50 to a position on the front of the tank so that it could be used in the anti-infantry role by the turret crew remaining in the tank. (Some early M4 cupolas actually had the Cal. .50 as a mount on the cupola itself, which could be rotated around). Better yet, some crews even replaced the Cal. .50 with a cal. .30, which arguably was a better anti-infantry weapon. I'm not sure what movie this is from (Who goes into battle with the 76mm in a travel lock?) but it does show the forward pintle in use. http://ronpalinkas.com/wp-content/uploads/2014/11/600px-TGK-M2HBShermanA.jpg

In any case, if you happened to have one of the M4s with the original mounting of the Cal. .50 on the turret rear, then absolutely it was not unheard of for someone to hang on the back and blaze away. http://www.theshermantank.com/wp-content/uploads/2016/01/pic1159997.jpg