(In which Juggernaut becomes so powerful, he smashes the fourth wall.)
I'm not sure what to make of this one. Well, that's not strictly true. I know exactly what to make of this: it's a huge disappointment. My confusion only comes in when I start trying to figure out how this happened.
Everything was going so well with the Beast's tenure as the star of this title. The whole spying/disguising idea was a solid enough base, and having Beast, Buzz and Linda all dancing around each other whilst two of them are hiding very different secrets was a lot of fun to watch. Add in some kind of apocalyptic threat alluded to by Beast's "mysterious" stalker, and the whole package was significantly above average by '70s standards.
And so someone, in their infinite wisdom, decided issue #16 would be the last appearance of Beast, and that the story would be a stand-alone Halloween tale about fighting the Juggernaut (because obviously, the Juggernaut is spooky and scary, and not at all just a big man wearing a copper chamber pot). We learn that the mystery woman was indeed Zelda, and that the terrifying threat to the globe is in Canada (which is hardly surprising, and note how she sought out Beast rather than wasting any time contacting Alpha Flight), but really, that's pretty weak. Since I was reading these issues in Essential Classic X-Men: Volume 3, I could at least find out what happened next (in Incredible Hulk #161), but for anyone who was only reading AMA, this must have been absolutely infuriating.
And even having read IHU #161, I still have no idea what happened with Linda Donaldson and the Secret Empire (the invaluable UXM.Net tells me it all went down in Captain America). It all just strikes me as a stupid way to run the title, especially since the first installment of the replacement storyline wasn't ready for issue #17 in any case, so they had to run an edited version of Beast's origin story from those UXM back-up strips we looked at a while back.
Anyway, with all that aside, this isn't exactly a bad story, but it's horribly pointless. Juggernaut returns from the dimension he was trapped in, tries to chase down Beast, and ultimately fails and seemingly dies. There's some nice ideas in there regarding the Beast's attempts to stop Marko, by testing out various scenarios - can electricity knock him out, will he sink in water, and so on - but that's about all. There's also several scenes in which the actual comic staff show up, helping Beast into town, inviting him to a party, and so on. I'm sure this all sounded hilariously clever when someone in the office dreamed it up but, much like Supernatural's "The French Mistake", it ends up being more wearying than anything else. 
This issue takes place over a single night. It's repeatedly confirmed that the story takes place on Halloween, which (as expected) causes us to change the timeline for the entirety of Beast's AMA issues, since they all happen over the course of a week or so.
I don't see any reason not to place this story on the day following AMA #15, then. The full revised timeline for AMA is below.
AMA 11. The Beast!: Sunday 26th October, 1980.
AMA 12. Iron Man: D.O.A.: Sunday 26th to Monday 27th October, 1980.
AMA 13. Evil is All in Your Mind!: Monday 27th to Tuesday 28th October, 1980.
AMA 14. The Vampire Machine: Wednesday 29th October, 1980.
AMA 15. Murder in Mid-Air!: Wednesday 29th to Thursday 30th October, 1980.
Friday 31st October, 1980.
The final day of the Mariel boatlift, an exodus of Cuban citizens to the United States that lasted over half a year, and which resulted in the emigration of an eighth of a million people.
"I'll be back, Earth! I'LL BE BACK!!"
 Also appearing briefly is Tom Fagan, a comic fan who organised a yearly comic-themed parade on Halloween in Rutland, Vermont (where this issue takes place) from the mid '60s until the mid '00s. In reading up about him I discovered this quote from Fagan's obituary in Comic Buyer's Guide (he died in 2008, just ten days before Halloween):
Riding on a float were Thor and Sif, along with the Norn Queen. The Red Skull hitched a ride on the float for no known thematic reason....It's lines like those that remind me how much I love the world of comic fandom.