1. To abandon asking
2. Not silently listening and not giving an attentive ear
3. Misunderstanding or wrong understanding
4. Lack of memorisation
5. Not disseminating and teaching it; for whoever accumulates [and keeps knowledge as a secret] and does not disseminate it, Allaah will afflict him by making him forget it- as a just recompense.
6. Not acting upon it, for indeed acting upon [knowledge] compels one to remember and ponder upon it, guard and examine it. But if he does not act upon it, he will forget it.

[Source: (غذاء الألباب) – page 33’ By Muhammad Bin Ahmad Bin Saalim As-Safaareeniy Al-Hanbali (rahimahullaah). Abridged & slightly paraphrased]