Vanishingly few non-embedded software applications can be fully guaranteed. The reason is that only a few top academic experts (Knuth, etc.) can write useful code that can be proved to contain zero bugs. Almost no other normal humans can do that for any fully featured and decent sized app. So every app you buy is almost guaranteed to have bugs, maybe just ones you haven't found yet.
Basically, you've got to check the reviews, and then take a risk if the number or frequency of reported problems seems low enough.
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