1) Unauthenticating Many Authentic Narrations:
Aboo Haneefah stipulated strict conditions for a narration to be accepted – very difficult conditions indeed, but he only did so because fabrication in hadeeth became a widespread phenomenon. During his time, Iraaq was the source of revolutionary and intellectual groups in the Islaamic world, and as such, it was fertile soil for fabrications and fabricators to thrive.
These factors impelled Imaam Aboo Haneefah to be more circumspect when accepting the authenticity of a hadeeth, and so he only accepted those hadeeth that were famous and widespread, and came from trustworthy people. In this regard he was more careful and strict than even the scholars of hadeeth which is why he deemed many hadeeths to be weak, which in the view of the scholars of hadeeth, were authentic and accepted.
2) Accepting Mursal Narrations:
Although he was strict in one aspect of judging the authenticity of hadeeths, he was more lenient in another: he would consider disconnected (mursal) narrations to be acceptable, but only if the one who narrated it was trustworthy.
Mursal narrations are narrations wherein one of the Taabi’een would narrate a hadeeth without mentioning the source from whom he took it.
Imaam Aboo Haneefah’s view in this matter is contrary to the view of the majority of Hadeeth Scholars, which led him to arguing issues based on hadeeths that were considered to be weak and inapplicable by others.
[Taken from "The Sunnah And Its Role In Islaamic Legislation" by Mustafa As-Sibaa'ee, p.492]