Maybe a boomer spawned before the tank.
Code:
public Event_TankSpawn(Handle:event, const String:name[], bool:dontBroadcast)
{
SetConVarInt(g_hBoomerLimit, 0);
for (int i = 1; i <= MaxClients; i++)
{
if (IsClientInGame(i) && GetClientTeam(i) == 3 && IsPlayerAlive(i) && GetEntProp(i, Prop_Send, "m_zombieClass") == 2)
{
KickClient(i, "releasing boomers"); //or forceplayersuicide. it has to be one that don't spread vomit into survivors.
}
}
}