One of the packs of additional quests included with the free download bundle for Daggerfall contains a FG quest to go kill six giants at the castle of a city in the region. They start to spawn outside the castle as soon as you talk to the local ruler inside.

I'm not certain if it's always the same town (Singcroft?), but on a few instances where I've taken this quest, the final giant fails to appear, and I'm stuck with the sound effect following the character until the time expires. On one occasion, I back-stabbed the second giant, who usually whistles up four tigers to join in, before he could do so, and I wondered if that broke the quest, but reviewing the way the quest is constructed convinces me it can't. I haven't had the same problem with invisible foes in any other quest, so I'm wondering if the final giant is getting spawned where there is no path to the player.

Anyone else had this happen, and have you any insights?

It's most likely that the extra quest is buggy. It's ok to leave the quest unfinished and wait for it's completion time to expire. Also you can get another fighters guild quest from a different guild in another town imideatly, just not from the same guy who gave you that quest (at least not until the quest expires).

On the other side, if you feel adventurous you can try using the template command line tool to convert that quest to an editable script text file, try to find the bug, then compile it back again. The template quest compiler and decompiler can be found here: http://slushpool.fortunecity.com/ .