With regard to karma one of the most cited Pāli passages is:
Cetanāhaṃ bhakkhave kammaṃ vadāmi - AN 6.63
Intention, monks, is what I call action.
In other words, only intentional actions are karma. On the other hand Jains considered that all actions whatever were karmic. They developed the idea that inactivity was the best religious practice, taking it as far as starving themselves to death.
This passage from Miln seems to contradict this important principle. It is very curious. I will have to give this a good deal more thought.
And I think it's fair to say that just because someone can come up with the plausible reading of this text by changing the meaning of the words somewhat, doesn't mean that this is what was intended. A more sound procedure is to exhaust the possibilities of the text itself, then look for a traditional commentary, before starting to make ad hoc assumptions that simply fit a desired interpretation. One always has to allow that the text simply does not make sense.
In checking the Pāḷi passage (Miln 84) (in the Burmese 6th council Ed.) I had to translate it anyway, So I'll include my translation for reference. If there are any supplementary questions about the translation, leave a comment, and I'll edit this answer to try to explain.
The question on doing evil knowingly or unknowingly.
Rājā āha "bhante nāgasena, yo jānanto pāpakammaṃ karoti, yo ajānanto pāpakammaṃ karoti, kassa bahutaraṃ apuññan" ti?
The King said, "Bhante Nāgasena, he who does an evil action (pāpakamma) knowingly (jānanto), he who does an evil action unknowingly (ajānanto); which has the greater demerit (apuñña)."
Thero āha "yo kho, mahārāja, ajānanto pāpakammaṃ karoti, tassa bahutaraṃ apuññan" ti.
The Elder said, "Majesty, he who does an evil action unknowingly, he has the greater demerit"
"Tena hi, bhante nāgasena, yo amhākaṃ rājaputto vā rājamahāmatto vā ajānanto pāpakammaṃ karoti, taṃ mayaṃ diguṇaṃ daṇḍemā" ti?
"By this [reasoning], Bhante Nāgasena, should we punish our royal son or royal minister twofold (diguṇa) for doing an evil action unknowingly?"
"Taṃ kiṃ maññasi, mahārāja, tattaṃ ayoguḷaṃ ādittaṃ sampajjalitaṃ sajotibhūtaṃ eko jānanto gaṇheyya, eko ajānanto gaṇheyya, katamo balavataraṃ ḍayheyyā" ti.
"What do you think, Majesty, if someone were to knowingly grasp a glowing iron ball, aflame, burning, or to unknowingly grasp it, who would receive the worse burn?"
Yo kho, bhante, ajānanto gaṇheyya, so [tassa] balavataraṃ ḍayheyyā" ti.
"Bhante, the one who grasped it unknowingly, he would receive the worst burn."
"Evameva kho, mahārāja, yo ajānanto pāpakammaṃ karoti, tassa bahutaraṃ apuññan" ti.
"Just so, Majesty. he who does an evil action unknowingly, makes the most demerit."
"Kallosi, bhante nāgasenā" ti.
"You are skilled, Bhante Nāgasena".
There is a lot here to digest. If Nāgasena is correct, why does Milinda say he should punish members of the royal court twofold (diguṇa)? I don't see the logic of this.
Does the analogy even work? Is it true that grasping a burning iron ball on purpose would mean one would be burned less? Who would grasp a burning hot iron ball on purpose anyway?
This is one of the most bizarre pieces of Pāḷi prose I've ever looked at!
I don't see how one can separate intention and understanding. Jānanto is just a very broad word for "knowing". So we must interpret yo ajānanto pāpakammaṃ karoti as "does an evil action without knowing that he does evil" and yo jānanto pāpakammaṃ karoti "does an evil action knowing that he does evil". The one who acts with knowing clearly must incur the greater consequences, but this text reverses the natural order. On face value it simply does not make sense.
It has been suggested by @RobM that this passage is referring not to the ethical quality of the action (and therefore of the vipāka it will generate), but to weightiness of the kamma. In fact I do not see any meaningful separation of these, since it is the ethical quality of an action that determines how weighty it is. Furthermore the passage itself is talking about who accrues the most apuñña or demerit. How can one separate puñña/apuñña from the "ethical quality of the action that generates it? An action generates puñña/apuñña in precise proportion to the kusala/akusala of the intention (cetanā) behind it. What's more the passage does not contain any word implying weightiness (garu, garutara, garutā etc). So although @RobM has produced a reading that is consistent with his understanding (and therefore pleasing), it is unrelated to the words he is attempting to interpret. This is an unsound exegetical methodology.
The exegetical strategy of all those answers so far which claim to understand this passage (which I do not) is that the words jānanto/ajānanto mean something other than what they mean. Either ajānanto means micchādiṭṭhi or it means garutā (weightiness) or something else that makes the passage make simple sense. In order to establish such a reading the person proposing it would need to show that this esoteric substitution was a regular feature of either the Milindapañha or the Pāḷi literature as a whole. One of the attractions of the Pāḷi literature is that on the whole it is quite straightforward. Metaphors and similes are occasionally reified by later tradition, but they stand out in the texts. So, on the face of it, an esoteric reading, where the text says one thing but means another would require some strong evidential support in the form of many quotations of similar substitutions. I don't see any evidence to date. So far I have not understood this text, but I don't believe that anyone else has either. "Don't know" must always be a possible, if unsatisfactory solution to any problem. And if one does not know then there is no shame in saying so. To over-ride unknowing and superimpose something familiar on a text to force it into a familiar pattern is not a good practice. At best it is a form of self-deception.