Hi gmat_perfect,

Good question. I believe this expression is used in one of the QG12 questions.

To explain why this is incorrect, we will place this expression in context: (Note that I am omitting "not" since its placement does not cause any ambiguity)

The construction continued until 2000.
The author implies that construction continued exactly till the year 2000.

Now lets insert "almost" at the right place.
The construction continued until almost 2000.
The author implies that construction continued till around 2000. He is not certain about the year and that is why he has used the word "almost". Thus, in this sentence "almost" modifies 2000.

Now lets place "almost" before until (as in the incorrect sequence of modifiers)
The construction continued almost until 2000.
As far as the timing is concerned, the author is sure about the year 2000 since he states "until 2000". So what is the use of the word "almost". "almost until" does not make sense at all. Similarly, almost cannot modify the verb - continued.

Thus, almost is misplaced in this version.

I hope this helps explain why this sequence of modifiers is incorrect.