What is a common phrase to describe something that changes while you are working on it without your being aware of it.
For example: you are adding comments to a document, and when you submit them you see that the doc was completely changed in the meantime and your comments are not relevant.
I would suggest moving target. Longman online (3) says:
a moving target something that is changing continuously, so that it is very difficult to criticize it or compete against it
'Capricious' sounds like more close since its more close to being 'arbitrary'.
'Volatile' is also nice to use here but if it is regarding your example of a document it must be 'Dynamic'
If you're looking for one with a negative connotation - you can say you are "building on quicksand".
In the UK we often accuse someone of "moving the goal posts" if they change requirements or conditions in a way that makes our efforts or arguments redundant. The same phrase can also be used in a less accusatory manner if events overtake us, but this is slightly less common, IME.
I would use "dynamic" which google defines as
(of a process or system) characterized by constant change, activity, or progress
If you wish to use it as a phrase then try "dynamic system", "dynamic process", "the dynamic nature of..." or "dynamic in nature"
"in flux" or "ephemeral" both seem useful here.
I've heard this situation called shifting requirements.
"It's hard to predict what the final product will look like, thanks to all these shifting requirements!"
We always called it a living document. Changes are ever happening and the status quo is always in flux.
The 1st word that comes to mind is "mercurial"- defined as changeable; volatile; fickle; flighty; erratic: Other fine words I would use would also be things like protean or mutable
If we are talking about something changing because you are working on it there are some good science terms namely the "Uncertainty principle", that observation alters the outcome. Alternately it could also be said to be "Schrodinger's Cat" a thought experiment where the item changes at the moment you observe it.
Evolving could be a suitable option.
“We Are Building a Plane in Mid-Air”
I'm not sure if this is a purely Australian expression, but here we often say that the "goalposts are shifting" in a situation where you can't achieve anything due to constantly changing requirements.
The phrase I would use in this case is "... right out from under me," with the leading part variable depending on the context. In your particular example, I would say something like:
Well, nuts, looks like this document got revised right out from under me.
Fluid (reference.com)
changing readily; shifting; not fixed, stable, or rigid:
Fluid (free dictionary)
fluid - subject to change; variable; "a fluid situation fraught with uncertainty"; "everything was unstable following the coup"
liable to change or fluctuate quickly: an unstable weather pattern.
(an unstable work/code/dev environment)
Posts submitted to StackExchange are subject to change and are continually evolving due to the editorial efforts of the community. That is the nature of the beast here at SE.
I have yet to see someone mention the most common phrase I would use to describe this: a ninja edit; that is, a change to something that happens while you are commenting on it.
I find it to be a very common saying on forums and message boards. Urban dictionary confirms this, if you consider that valid:
A ninja edit is an change made to a published post or article (typically on a bulletin board) that preempts the first response made.
'The ground is shifting under my feet' is an expression that I think most closely reflects the situation described of a document being edited while you are editing it.
'Shifting goalposts' is not quite analogous. More suitable for the 'boss' changing what they want the document to say when you present it for review.
Try quicksilver. It has the element of mercurial (being a colloquialism for the element mercury) but also contains the uncertainty principle idea as it "runs away" when you try to touch it.
So, you could say "editing quicksilver" or "a quicksilver task" or "the quicksilver meaning" or "a page of quicksilver paragraphs".
It also has a kind of infinite regression quality, since it was originally the name of the mercury / tin alloy used on the backs of mirrors to cause reflection.
How about transitory, fleeting, or fugacious?
You might consider evolving, mutating or mutable.
Some additional phases that I have heard in the past include:
Dancing on shifting sands
Running on water
Snatching at fog
Trying to nail down a shadow
is the phrase that comes to my mind.
The phrase which springs to mind for this to me is "Scope Creep":
A few phrases that capture the time / out-of-step component of your question: "a day late and a dollar short" or "always behind the eight-ball" or "I zig when everyone else zags.
You can probably say, by the minute, transient, on the turn. Since you asked for a phrase the first and last should do.
clusterflux
also
like nailing jelly to a tree
I'd say : Feels like working on a chameleon.
requirementhaha. They always change when you are working on one.