Basically you can set conditionals before running text. If you're using twine (the current one) you're more then likely using Harlowe, a style of twine. I establish the spoiler warning in its own passage, to make a variable (in my case, it's $spoiler) 1. Then, before any big choices, i can write the following:
(if: $spoiler is 1)[SNAPE SNIFFS DUMBLEDORE]
I have to do the text and stuff myself, but it only shows up if the variable value is what i specify. (Same reason why the folks who got bad-ended on tour don't show up after their bad end)
