Original report.

There's a lot of similar cards; there are different +1/+1 counters for Dwarven Weaponsmith (card_instance_t::counters; shared by all cards programmed in C), Ashnod's Transmogrant (card_instance_t::counters5), Fungusaur, Rock Hydra, Sengir Vampire, Khabal Ghoul, Citanul Druid, Tetravus, and Triskelion (all card_instance_t::special_counters). Whirling Dervish used special_counters also until it was moved into C.

Generally, I've made new cards that care about whether +1/+1 counters are present check all these special cases; the problem is that none of the cards themselves check +1/+1 counters from other sources. I don't think that's a particular problem for any of the others except Tetravus and Rock Hydra.

Manalink made them all use the same sort of +1/+1 counters (all in card_instance_t::counters) up until I rewrote the entire counter engine last year; I hadn't been planning on the intermediate step, but it looks like it's going to be necessary at least for these three cards.