[kaev]

Targeting bug is old. When it happens (usually after gaining target via /assist or "target nearest npc" hotlkey when mob is out of visual range), there are 2 effects:
- your character's melee/range and secondary (i.e. bash/kick) attacks will not happen (no damage, no threat generation, no animation), and
- the mob's hp bar will not update when others hit it or it is damaged by yours or others' spells (including a DS on you)

Workaround is as whydothis says: you must lose target on the mob (either use escape to drop target or change to another target) and retarget it. It does not work to double-tap tab in this situation, tabbing to yourself and back to the target will not clear the bug. You must click on the target, assist somebody else, or use a /target command after losing/changing targets.

@nilbog:
it is easy to replicate this bug in a zone with reduced visual range (Qeynos Catacombs, for instance) by using "target nearest NPC" to get target on a mob you can't see yet. A pathing mob like Argyn Shieldstan who has a long straight portion of his path with no other mobs in los, for instance:
1. face the spectre spawn area from near Drosco's spawn point when Argyn is out on his stroll,
2. with Argyn out of sight, hit 'target nearest npc' until Argyn appears on your target, which will occur before Argyn comes into view,
3. at this point you may cast on Argyn, if your cast does damage his hp bar will not update altho he will be affected by the spell and will agro, when Argyn closes to melee range your melee attacks will not have any effect.
4. if you lose/change target and then retarget Argyn the bug is cleared