Where could rviz error "Frame [/example] does not exist come from?

edit

Hi,

I'm a newbie to rviz/ROS and I've been trying to overlay markers on a robot URDF in rviz. I'm getting a curious error where I create a series of 30 markers in a loop, and sometimes the last 10 markers produce an error and sometimes #20-23 do.

The basic setting I'm using is

======= URDF File =========

<link
  name="base_link">
  // Other urdf origin/mesh things. The link appears fine, so that wouldn't be the problem
</link>

<joint name="dist2_tactile_joint" type="fixed">
  <origin xyz="0 0 0" rpy="0 0 0" />
  <parent link="base_link" />
  <child link="example1" />
</joint>

<link name="example1"/>

======= C++ File =========

visualization_msgs::MarkerArray marker_array;
char fid[20];
for (int i=0; i<3; i++) {
    sprintf(fid, "/example%d", (i+1));
    for (int j = 0; j<10; j++){
        visualization_msgs::Marker marker;
        marker.ns = "contact_markers";
        marker.id = j;            // Goes from 0-9 for each frame_id
        marker.header.frame_id = fid;
        // Set position, color, orientation stuff
        marker_array.markers.push_back(contact_marker);
    }
}
pub.publish(marker_array);      // To the topic visualization_marker_array

I'm getting an error in rviz (not in rqt_console) every couple frames or so, saying

example_markers/0 -- For frame [/example3]: Frame [/example3] does not exist

The weird bit to me is that it often throws an error for example3/*, rarely throws an error for example2/8, and doesn't throw any other errors. Because I'm creating the markers in a loop I don't see how they could be different.

Help much appreciated!