| Interesting experiment. You would not have gotten away with it at higher levels where the total number of modifiers gets difficult to manage though. While a roll of a 6 is probably a failure for most players, if that player has a high stat modifier, and some magic at work to buff the number, as well as feats optimized to the action, you could get caught.
Unless of course you are accounting for who is doing what when deciding the difficulty range for the character.
But relying on how well you know your PC's characters can backfire. I am generally hard pressed to recall the specifics of any of my players character sheets.
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