There's an expression I've read many times in slightly variant forms: "איזה צרה שלא תבוא" or "על כל צרה שלא תבא" or the like. It means something like "any tragedy that should not happen" and it's used in running text like "and in case of any tragedy that should not happen, do such-and-such", as in Kitzur Shulchan Aruch 121:11. How do we understand this phrase? I can think of two possibilities:
What we really mean to say is "על כל צרה שתבא", "in case of any tragedy that may occur", but because Judaism holds ברית כרותה לשפתים, words have effects, we don't want to say "צרה שתבא", literally "tragedy that will occur", so we change it to "צרה שלא תבא", literally "tragedy that will not occur".
What we really mean to say is "על כל צרה", "in case of any tragedy", and, because we're mentioning tragedy, we stick in a short prayer to God: "שלא תבא", "that should not occur".