I am reasonably certain that I have seen instances of this:
I post a reply to a thread and (eventually) see it appear at the bottom of the screen, immediately following the post to which it was a reply. I go back to the thread later and there’s somebody else’s reply sandwiched in between my post and the one to which I was replying.
Can this happen or am I imagining it and just need to refine my observational skills?
Hmm. In theory, it can’t happen.
With this server, I wouldn’t be surprised if a suffocated hamster got wedged in between two posts accidently. My theory is that you posted at the same time as the wedged post, and the server just copied you on your post the first time 'round, as the other post was still going through as well. Later (perhaps after another post), it had indexed succesfully, and your post lost the race.
Does that make sense?
Sounds like a simulpost that didn’t ‘take’ until the page was refreshed. The first time you saw the post, the other post was made but not yet displayed; when you returned to the thread, it was displayed.
I’ve seen this occur with my posts a few times also. You’re not alone in this crazy universe after all.
Another possibility is mod surgery. You know how sometimes, a person intends to post a reply to a thread, but accidentally posts a new thread instead? With the new version of the software, the mods can now merge the threads together to fix it. I think that this might result in what you’re seeing.
Then couldn’t we help the software by putting a hint into the HTML code where it should insert the gerbil? 
I had a post wedged in front of another one which appeared while I was still previewing, and ‘complained’ about it in this thread. Looking at it again, it took me at least a couple of minutes editing my post, and the board says it thinks there have been 18 minutes between the two. The time/order difference can’t really be explained with overlapping post database updates only. I think at least some of them are genuine glitches.
I’ve seen something similar. I recently (late at night) ran a search looking for all Threads I’ve posted to since the Dawn of Time. It made for some interesting, ego-satisfying and embarassing reading.
(Side Note: You know who was the first person to respond to my first thread? Satan. Boy, I miss him. 
)
Anyway, looking at those old threads revealed some interesting quirks. For example, IMG tags, QUOTE tags, LINK tags and other formatting commands have changed, so there’s lots of gobbel-dy-gook in there.
But, stranger still, the posts in some of the threads looked out of order. I’m certain that in at least one of them, my OP was stuck somewhere in the middle. Wierd.
Of course, I can’t reproduce it. I’d never find it again. Believe it or not!
sdimbert - when we converted from UBB to vBulletin, the order of posts in the threads was not preserved. So any thread created under UBB will have posts out of order.
Yeah, sdimbert, the conversion from UBB to vB caused the old glitches you note. Post sequencing in threads was scrambled due to, I believe, some previous clock/date troubles. Also, all the old UBB coding was converted to text.
somewhat related… but didn’t seem candidate for a whole new thread… so i’ll mention it here…
a few days back, i posted a reply to a thread in GQ using the quick reply box, before being logged into the SDMB. it took me to the usual “you are not logged in” page. so i logged in, backed up and submitted. then when i checked the GQ page, i saw the thread i posted in saying “last posted by Unregistered”!
i was about to post a thread in atmb about it, but when i refreshed it came up correctly as “last posted by xash”
so weird things do happen…
It can happen, if the Board is really, really, really bogged down, where the SQL Insert time is not updated - that is, a post has been assigned a spot in the database, but its actual connection to the thread view is “pending”. So, yes, it can happen, and it’s harmless, as long as no data is lost or corrupted.