A common trap for beginners at P'saq is to over-apply generalities without regards to various nuances or exceptions.
For Example see SA YD 69 and in particular the N'qudot Hakkesef "maaseh ba l'yadeinu"
Here the Shach dismisses the rule "s'feiq d'rabban l'kulah" He knows it does not apply because "all the posqim require" something in addition, in this case "mei'siach lefi tumo" or "yotseit v'nichnas" - the latter a function of mirtat.
Why do Posqim require this additional evidence or Mirtat required? Why doesn't the s'feiq d'rabbanan suffice? - Because as the Shach here points out
«Vkchol heicha d'itchazeik issur lo amrinan "s'feiq d'rabban l'qula"» and as he elabourates further in YD 110:63.
In my experience, many rabbinical students and new rabbis are prone to repeat the broad rule and to ignore the second rule. Which is perhaps the reason for studying issur v'heter in depth, in order not to come up with over-simplistic Halachah based upon superficiality.