Halberds cannot attack over 1-tile-wide objects
What I did: Attacked a TzHaar citizen, led it to a vent, trapped it on the other side, attacked it. Also, attacked a Ram in Lumbridge, ran out of the pen, closed the gate, attacked it.
What I expected to happen: I hit the NPC with the halberd, potentially doing damage.
What actually happened: I ran around the vent to attack the TzHaar citizen. Also, I did not swing at the Ram at all.
IDs of related NPCs/items: 3202 (Rune Halberd)
2009-era source (if relevant): https://runescape.wiki/w/TzHaar-Ket?oldid=892679 - Jan 2009 mention of specifically attacking TzHaar with halberds over the vents and mining rocks.
Screenshots or video: Unless you want to see me dying to a TzHaar or staring at a Ram doing nothing, a screenshot probably won't help.