So it appears there actually are some more left-over test elements.

hot_potato
hot_potato_catcher
hot_potato_socket
hot_potato_spawner

Found these with the "dumpentityfactories" command. They work basically the same as the glass futbol entities, except that hot_potatos explode on contact with surfaces. They still use futbol models, and can be carried by the player. When put into and taken from the socket/catcher (those entities seem to be identical), the prop_button ding noises play.

Interesting find!

Some cvars that seem to be related -```
"hot_potato_end_color" = "255 106 0 255" game cheat                              - The ending color of the hot potato, before it starts the final explode sequence.
"hot_potato_explode_on_fizzle" = "0" game cheat                                  - If the hot potato should explode when it fizzles.
"hot_potato_explosion_damage" = "25.0f" game cheat                               - The damage of the explosion for the hot potato.
"hot_potato_explosion_damage_falloff" = "0.75f" game cheat                       - The percentage of damage taken at the edge of the explosion.
"hot_potato_explosion_debug" = "0" game cheat                                    - Debug the explosion of the hot potato.
"hot_potato_explosion_magnitude" = "0" game cheat                                - The magnitude of the explosion for the hot potato.
"hot_potato_explosion_radius" = "200" game cheat                                 - The radius of the explosion for the hot potato.
"hot_potato_flash_duration" = "1.0f" game cheat                                  - The flash duration of the hot potato, right before it explodes.
"hot_potato_flash_end_color" = "255 0 0 255" game cheat                          - The final color of the hot potato, right before it explodes.
"hot_potato_flash_start_color" = "255 255 0 255" game cheat                      - The start color for the hot potato flashing before it explodes.
"hot_potato_flash_start_time" = "3.0f" game cheat                                - The time before the hot potato explodes when it start to flash.
"hot_potato_hit_breakables" = "1" game cheat                                     - If the hot potato should hit breakable entities.
"hot_potato_phys_mag" = "100" game cheat                                         - Magnitude of physics force for an hot potato
"hot_potato_phys_rad" = "45" game cheat                                          - Magnitude of physics force for an hot potato
"hot_potato_physics_punt_player" = "1" game cheat                                - Physically perturb the player when the explosion hits them
"hot_potato_start_color" = "255 255 255 255" game cheat                          - The starting color of the hot potato.
"hot_potato_use_cooldown_time" = "0.7f" game cheat                               - The cooldown time for the use key after the player picks up the hot potato.

```
"final explode sequence"? hmm

There's an old map called antimatter that I think uses them unless it's just source ents. It also has a futbol reskin.