If a product is marked as ''Oops'', it is because this product has inferior quality to what we see as Standard, and therefore is discounted.
Oops stickers (available in bags of 5 sheets, may include things such as: printing imperfections, cuts that are not deep enough, cuts that are too deep, miss cuts (never cutting the actual image), dirty sheets (random ink drops on the non-sticker area of the sheet).
Oops washi tapes can either show up as not sticky enough (halloween 2019 tapes), or a random printing imperfection (sunflower fields 2020).
Discount on these will depend on how it affects product usage (washi that is less sticky will have a bigger discount than a washi that has a small printing mistake).