I have to kill rockslugs, and whenever I deplete their hitpoints and try to use a bag of salt on them, I get the message "Someone else is fighting that." What the heck?

picture? just incase if i can help u solve this problem

Alright, here's one attempt. As you can see it's clearly low enough HP wise, and there isn't even anybody else in the area to fight it.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

And after that they kinda stop moving. Same one a couple of minutes later.

 

 

 

 

 

 

 

isnt that a burthorpable task? if so that can be a big problem, means people cant complete their tasks. i suggest you report the bug to jagex

Well after messing around a bit it turns out you only can't do it while they're at zero- if I use the salt on them before their HP is totally depleted, or give them enough time for the red HP bar to disappear, it works. However, I'm pretty sure it hasn't always been like that.