The gemara (Avodah Zara 22) quotes R’ Shimon ben Elazar that a field should not be rented to a kusi because the kusi will work the field on Chol haMoed, and since the field is known to the public as belonging to a Jew, there is a potential issur of maris ayin. The gemara writes that there is an additional potential issur of lifnei iveir not mentioned by RSb”A, assuming the kusi is a true ger. According to Tosfos, the issur of doing work on chol hamoed is an issur derabbanan, and from this gemara we can conclude that lifnei iveir applies even to causing a person to violate issurei derabbanan. Ramban, however, writes that there is no issur of lifnei iveir on issurei derabbanan, and concludes from this gemara that the issur of working on chol hamoed is an issur d’oraysa.
The Minchas Chinuch (232) questions why lifnei iveir, which applies even to mundane matters like giving bad business advice, should not apply to causing someone to violate an issur derabbanan. Perhaps one can distinguish between bad advice or issurei Torah which are inherently detrimental, and issurei derabbanan, which are prohibited because they are an act of rebellion against the authority of Chazal – if done inadvertently, no rebellion against Chazal’s authority has occurred, and no issur has been caused. The dispute between Tosfos and Ramban may revolve around this very point – is an issur derabbanan done b’shogeg defined as a ma’aseh issur or not?
To be fair, I made two other assumptions here: 1) lifnei iveir only applies when the person being led astray acts b’shogeg, but if he/she intentionally chooses to act in a manner she/she knows is wrong or intentionally follows what she/she knows is bad advice, there is no lifnei iveir (violating a derabbanan b'meizid would be an act of rebellion and my hesber fails); 2) lifnei iveir must lead to a ma’seh issur; an attempt to lead someone astray which fails because it is not acted upon or no issur occurs is not technically a violation. Both points are raised by various achronim.
One could possibly explain the machlokes Tosfos and Ramban as relating to how to understand lifnei iveir – is it a broad overarching prohibition, or is it a way of extending each issur on an individual level. If the latter is the case, perhaps extending issurei derabbanan amounts to a gezeirah l’gezeirah type of situation. This approach leaves the M.C.’s question unanswered.