Freeciv

found_new_tech() has:
/* Send all player an updated info of the owner of the Marco Polo * Wonder if this wonder has become obsolete. */
if (0 < had_embassies[i]
&& 0 <= get_player_bonus(aplayer, EFT_HAVE_EMBASSIES)) {

Fist line checks if player used to have the effect.
Last line is supposed to check if the player no longer have the effect (wonder has become obsolete). However, it's true when ever effect value is zero (disabled), or above (enabled). The only time it's not true when the effect value is below zero (disabled).