About 1 November 1864, Samuel Mock was helping carry logs to build winter quarters. (He describes it as "build[ing] a tent", which suggests they were building log walls, and using their tents for the roof.) The other soldiers dropped their end, which knocked him down. They carried him to his tent, and he then discovered that his back was injured. The regimental records do not have any record of this, and the only treatment he received was in the camp hospital.
The regiment went on Warren's raid on the Weldon Railroad from 7 to 12 December. Bowen later reported that he saw Mock on the return from that raid, and he was unable to walk normally, and unable to carry anything but his blanket. Perhaps this explains why the November/December 1864 muster roll reports him as owing the government $23.25 because he had lost his gun.
Despite this injury, he remained with the regiment until 30 May 1865, when he was discharged in accordance with War Department general order 94.
Mock's description
"That while a member of the organization aforesaid [sc. company B of the 91st PA], in the service and in the line of duty at near the Yellow House, in the State of Virginia on or about the 1st day of November 1864, he injured his Back or spine in the following manner to wit he and [illegible word] of his comrades were carrying a log to build a tent and he had one end of the log and his comrade the other and they let their end fall, and he was knocked down, and he was carried to his tent and their [sic] discovered his back was injured causing a disability from that time to this [four illegible words]. That he was treated in hospitals as follows: none except in Camp Hospital"
("Declaration for an original invalid pension", by Samuel S Mock, 26 June 1880, Bedford, Pennsylvania)
Bowen's description
"I was a member of Co "C" 91st Regt Pa Vol Infantry and was well acquainted with the above named Claimant Samuel S Mock of Co. "B." Same Regt and I well remember that some time between the first + fifteenth of December 1864 our Command was on a raid on the Weldon Rail Road, and I saw claimant [sc. Samuel Mock] on said raid, and he was suffering from effects of an injury which he had received in his back some time in November 1864. When I saw him on the Weldon Rail Road raid he was unfit for service, was unable even to carry his gun or anything except his blanket and could only walk in a stuping [sic] position. I also saw him when he first recd the injury to his back in Nov. 94 + I asked him what ailed him and he said he had recd an injury to his back by assisting in carrying logs for winter quarters, by his comrades [sic] letting the one end of the log fall while he had the other end. When I saw him at the time of the Weldon Rail Road raid we were returning to our quarters near the Yellow House in front of Petersburg Va and he was scarcely able to get along, and I feared that he would not be able to reach Camp again."("General Affidavit", by Alva Bowen, 2 February 1895, Bedford County, Pennsylvania, in Samuel S Mock's pension file)