Why is a transform not available after waitForTransform?

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Hello ros-users,

I have strange issue with tf-transforms. I use laser_geometry::LaserProjection to project laser scans to point clouds, but I assume the problem is related to tf. The code-snippet is from the scan callback function.

if(!gTfListener->waitForTransform(scan->header.frame_id,
                              "camera",
                              scan->header.stamp,
                              ros::Duration(2.0),
                              ros::Duration(0.01),
                              &error_msg))
{
    ROS_WARN("Warning 1: %s", error_msg.c_str());
    return;
}
try
{
    gProjector->transformLaserScanToPointCloud("camera", *scan, cloud, *gTfListener);
}catch(tf::TransformException e)
{
    ROS_WARN("Warning 2: %s", e.what());
    return;
}

After a few seconds of running, Warning 2 is raised on the console for like 1 out of 4 scans with this error message:

Warning 2: Lookup would require extrapolation into the future. Requested time 1412757571.359567610 but the latest data is at time 1412757571.357117891, when looking up transform from frame [laser_link] to frame [camera]

How can this happen, after waitForTransform obviously succeeded (returned true)?

Thanks in advance, Sebastian