If your spec is good, it's rare, because you're most likely to see a showstopper appear 10% or 20% of the way into making the 80% solution - and that's early enough that you can rearchitect and continue.
The cases where it becomes almost impossible tend to come from the "reuse something written to a different spec" projects where you're forcing things through the wrong architecture. The warning against over-focus on genericity at the end of the article is directed at exactly this - when you aim for generic you can often find yourself building to a spec without concrete goals, thus the actual problems are poorly dealt with.
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If your spec is good, it's rare, because you're most likely to see a showstopper appear 10% or 20% of the way into making the 80% solution - and that's early enough that you can rearchitect and continue.
The cases where it becomes almost impossible tend to come from the "reuse something written to a different spec" projects where you're forcing things through the wrong architecture. The warning against over-focus on genericity at the end of the article is directed at exactly this - when you aim for generic you can often find yourself building to a spec without concrete goals, thus the actual problems are poorly dealt with.