Inconsistent upload/PUT access to Amazon AWS S3 with custom permissions

Question

I have an application that uploads videos to an S3 bucket, and then creates a custom policy to allow another user (for the Zencoder service) to grab the files, and upload the transcoded files back into the bucket.

Below is the current custom policy I give to the user during transcoding. Basically I give full read permission to the entire bucket, but I only allow the user to PUT files into a specific nested folder.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowUserToListContentsOfBucket",
      "Effect": "Allow",
      "Action": [
        "s3:ListBucket",
        "s3:ListBucketMultipartUploads",
        "s3:GetBucketLocation"
      ],
      "Resource": [
        "arn:aws:s3:::MY-BUCKET"
      ]
    },
    {
      "Sid": "AllowUserToListContentsOfBucketFolders",
      "Effect": "Allow",
      "Action": [
        "s3:ListBucketMultipartUploads",
        "s3:GetObjectAcl",
        "s3:GetObject"
      ],
      "Resource": [
        "arn:aws:s3:::MY-BUCKET/*"
      ]
    },
    {
      "Sid": "AllowUserS3ActionsOfSpecificFolder",
      "Effect": "Allow",
      "Action": [
        "s3:PutObjectAcl",
        "s3:PutObject"
      ],
      "Resource": [
        "arn:aws:s3:::MY-BUCKET/some/nested/folder/*"
      ]
    }
  ]
}

This works for the most part, but in the ~1,000 files transferred over by Zencoder, there's usually one or two that fail with a 403 Forbidden error. I'm not sure why, since files were correctly transferred both before and after the error.

Is there any reason Amazon AWS S3 / IAM would send a 403 Access Denied when such a permission is provided?