Similar to this is Quirrel's mannerisms at the very start of his first class, meant to get you to think he'll be the typical Quirrel before he bursts into a confident diatribe, but people have speculated on that one, too.

For the record, it never occurred to me it could be Riddle's diary, and yet I assumed it was a horcrux because it was indestructible. I was probably pattern-matching with the Pioneer probe, which Quirrelmort also said he had spelled to be indestructible and this was, indeed, a Clue that it was a horcrux.

Also, in canon, a horcrux can possess you if you become emotionally attached to it. The diary seemed calculated to become a prized possession of Harry's, and that when Harry learned Latin he could well be in for a nasty shock...

Perhaps I should have updated when it was never mentioned again. Hmm.

As for myself, I quite liked the inclusion of Roger Bacon's diary as part of the background. Though it did lead me to expect a bit more information on 'leakage' between the magical and mundane worlds.

The indestructible diary was meant by Eliezer to be a red herring for the duration of a couple paragraphs, until the readers were told it was the diary of Roger Bacon, not Tom Riddle.

Even that's strange though. I didn't seriously entertain the thought that it was Tom Riddle's diary or a horcrux. And then it being Roger Bacon's attempts to apply science to magic was super-awesome - it was sort of a Chekhov's Gun and I kept expecting Harry to gain some sort of insight or initial boost or secret knowledge from reading it.

That's probably a fair analysis.

I was thinking the same thing. The things he thinks should be obvious by now (such as the quirrel/voldemort connection) ought to be made explicit in an appropriate point-of-view so we can puzzle over the things that he wants the reader to be puzzling over.