The Rat of Destiny

[Screen Title: SAN FRANCISCO]

[The camera pans to San Francisco Bridge, all the way to a self-storage facility with a sign saying "U-STORE It, SELF STORAGE", with its interior filled stored stuff and junk until it spots Luis' van from "Ant-Man and the Wasp", behind a metallic fence with a label named "LANG". A rat crawls over the van's rear windshield and accidentally activates the controls for opening the Quantum Realm, haphazardly activating the Quantum Realm. The rear door busts open, flinging Lang outside the van.]

SCOTT LANG: 

[grunts as he pushes away a cushion out of his body, and deactivates the helmet, but still grunting in pain, as he sweeps away sparks from an electrical failure out of his suit, then try to stand up.] 

What the hell?

[He manages to stand up, as another electrical failure sparks out of his gauntlets, and now looks around in a confused look.]

SCOTT LANG: 

Hope?

[Inside a security office, a security guard (Ken Jeong) reading a book 

as he looks upon his security screen, seeing something going on in one of the storage facilities. In the cameras, it shows Lang in his casual clothing, shouting, waving a sign with the word "HELP" written on it.]

“A rat crawls over the van's rear windshield and accidentally activates the controls for opening the Quantum Realm, haphazardly activating the Quantum Realm.”

The LOTR wiki lists four times Eru Ilúvatar intervened in the history of Arda:

Did Eru Ilúvatar trip Gollum?

The LOTR wiki lists four times Eru Ilúvatar intervened in the history of Arda:

1 : Creating Elves and Men

2 : Removing Aman from the spherical Earth

3 : Resurrecting Gandalf

4 : Making Gollum trip

In a letter written by Tolkien, he stated that Eru again intervened, this time in the Third Age, causing Gollum to trip and fall into the fires of Mount Doom while still holding the One Ring, thus destroying it.

Is this correct? If so, what letter is this, and exactly what did Tolkien say?

Yes, as we can see on Letter #192, Eru certainly took over after Frodo was done with the assigned task.

Tolkien mentions that Frodo did take the Ring to a certain point (where no other being could) and then another power took over to decide the fate of the Ring.

“Frodo deserved all honour because he spent every drop of his power of will and body, and that was just sufficient to bring him to the destined point, and no further. Few others, possibly no others of his time, would have got so far. The Other Power then took over: the Writer of the Story (by which I do not mean myself), 'that one ever-present Person who is never absent and never named' (as one critic has said). 

See Vol. I p. 65. 2 A third (the only other) commentator on the point some months ago reviled Frodo as a scoundrel (who should have been hung and not honoured), and me too. It seems sad and strange that, in this evil time when daily people of good will are tortured, 'brainwashed', and broken, anyone could be so fiercely simpleminded and self righteous.”