Well, that still doesn't explain the "one or more tasks" line on page 12, but I suppose that, by this point, I should know to expect redundant/confusing wording from FFG....

Not sure where you are seeing that line at on page 12.  My rulebook only has 12 pages and the last page finishes telling you how to combat the Goo.   Nowhere in that description does it say that.

Sorry, page 11.  Not sure how I got 12.  The third paragraph of page 11 begins with "If a player successfully completed one or more tasks on a monster marker," thus indicating (seemingly) that there can be more than one task on a monster marker.  Unless I'm mistaken, there's no way for monsters to gain extra tasks other than Shub, whose ability apparently does not really add a task.

well sometimes writers/developers use loose language to leave open room for expansions later.  So who knows, maybe there will be bad ass monsters in an expansion that have more then one line/task.

Baenre said:

I tend to think this is correct, that the Terror should be added to the task, not add a terror task.  The monster would actually be harder to defeat with the result added to the task than with a second task.

 So, to be clear when you defeat a monster, you cannot remove that monster(and the dice used to complete its task)until you have either completed the adventure or failed the adventure?

that is to say, you cant defeat a monster that was added to bottom of an adventure and immediately remove it and take its rewards thus freeing the dice used there and add those back into your pool and continue trying to complete the rest of the tasks on the adventure?

ALLular13 said:

Correct.  

"Once an Investigator resolves an Adventure card with a Monster marker on it, he removes all dice from the marker and takes it as well as the Adventure card..."  

and later, for failing to resolve the Adventure:  "he still gains the Monster marker, removing all dice from the marker,..."