Scholars have long proposed a lost verse of Grímnismál said to occur between stanzas 31 and 32. This is clearly based on a close reading of Snorri's Edda. 

In Gylfaginning 16, Snorri writes:

It is demonstrably a scholarly creation based on a close reading of Gylfaginning 16, and attributing Snorri's text with undue weight. This just goes to show how strong an influence Snorri’s Edda was in interpreting the poems of the Poetic Edda was more than a century into modern Eddic studies.
 
The lost verse, once proposed as a theory, was accepted into subsequent English translations and commentaries, and held for almost 80 years.  Since the 1960s, scholars no longer mention it.

Of this verse, Gudbrand Vigfusson, in his Corpus Poeticum Boreale (1883) writes:
 
“24-29. Three roots stretch three ways under the ash Ygg's-steed. Hell dwells under one, the Frost Giants under the second, mortal men under the third.”
 
“…Here one verse is dropped, of which Edda (Snorri) gives a paraphrase.”
 
Vigfusson provides a translation of the verse he refers to here as:
 
 “An eagle sits in the branches of the ash Ygg's-steed, called . . . . , and between his eyes sits a hawk, that is called Weather- pale.”
 
In his Kommentar, p. 201, R.C. Boer supplied another version of the  missing verse. He writes:
 
Nach 31 fehlt eine strophe. Das ergibt sich daraus, dass von dem 32 erwähnten örn schon vorher etwas gesagt sein musste und dass Snorri für seine prosa (Sn.E I, 74) offenbar eine poetische quelle benutzte [s. den text band z. st.]. Hieraus denselben selbstverständlichen schluss zu ziehen, wie Mogk, Müllenhoff und Gudbrandr Vigfusson, haben Detter-Heinzel und Neckel nicht gewagt [s. auch Boer II, 66] Die strophe lässt sich sogar aus Snorres umschreibung mit annähernder sicherheit wiederherstellen:
  
"After 31 a strophe is missing. The results from the fact that of the 32 previously mentioned örn (eagle) something had to be said for his prose and that Snorri (Sn.E, 74) apparently used a poetic source [see the text volume for st.]. From drawing this same obvious conclusion as Mogk, Müllenhoff Gudbrandr and Vigfusson, and have not dared to see Detter-Heinzel and Neckel [Boer also II, 66] The strophe can even be recovered from Snorri paraphrase with a degree of safety:"

Örn sitr á asks limom
Es vel kveþa mart vita;
Ögler einn hönom augna í mille
Veþrfölner vaker.
 
Diese herstellung, die Gering in den text seiner.
 
"This creation, Gering provides in his text."
  
In 1908, Olive Bray inserted a different version of this verse into her translation of Grimnismál:
 
“An eagle sits in the boughs of the ash,
knowing much of many things;
and a hawk is perched, Storm-pale, aloft
betwixt that eagle’s eyes.”
 
She remarks:
 
“Not found in the MSS, but reconstructed from the prose of Sn.E (by Gering).”
 
And of this theory, in 1923, Henry Bellows in his translation of The Poetic Edda writes:
 
“After this stanza it is more than possible that one has been lost, paraphrased in Snorri’s Edda thus: “An eagle sits in the branches of the ash-tree, and he is very wise; and between his eyes sits the hawk who is called Vethrfolnir.”

Then, remarkably, in 1962, Lee Hollander in his translation of The Poetic Edda issued the final words on the subject:

32. (An eagle sitteth on Yggdrasil's limbs,
whose keen eyes widely ken;
'twixt his eyes a fallow falcon is perched,
hight Vethrfolnir, and watcheth.)
 
"This stanza is lacking in the original. We are able to reconstruct it from Snorri's close paraphrase ("Gylfaginning," Chap. 15). The eagle and the falcon possibly symbolize the watchfulness of the gods."

After this, the issue quietly faded away.