In MN 56, when hearing that his disciple Upāli had been converted by the Buddha, the Jain leader Mahāvīra is depicted as being practically apoplectic. He reprimands Upāli with some memorable similes, similes which, however, I think have been misunderstood.
The point here is that Upāli went to the Buddha in order to teach him, to disabuse him of his mistaken ideas and convert him to the real truth, Jainism. But he came back himself converted.
All of the translations (Chalmers, Horner, Ñāṇamoḷi/Bodhi) essentially agree. Here is Ven Bodhi’s rendering:
Seyyathāpi, gahapati, puriso aṇḍahārako gantvā ubbhatehi aṇḍehi āgaccheyya, seyyathā vā pana gahapati puriso akkhikahārako gantvā ubbhatehi akkhīhi āgaccheyya;
Just as if a man went to castrate someone and came back castrated himself, just as if a man went to put out someone’s eyes and came back with his own eyes put out …
The problem here is that it depends on reading the two quite different words hāraka and ubbhata as having the same meaning. But the basic sense of hāraka is “bringing, delivering”. The PTS dictionary includes “removing” in its definition, but this is a minority sense.
Based on a range of sources in Tibetan, Sanskrit, and Chinese, Ven Anālayo develops a case that the metaphor is about someone who goes in search of something (either eggs, an alternate sense of aṇḍa, or myrobalan, an alternate meaning of akkhika) but comes back having lost their own balls or eyes. While this may work for the other texts, in the context of the Pali the whole reading seems odd and artificial to me, and I can’t see how the metaphor applies. Upāli wasn’t searching for something, he knew what he had already. Anyway, hāraka doesn’t mean “search”.
Let’s come back to the basic metaphor. Does Mahāvīra send out Upāli on a mission to “castrate” or “blind” the Buddha, even if only metaphorically? Why would he do that? It doesn’t seem very nice, and certainly not something that a compassionate spiritual leader would do. Even when acting in a hostile manner, religious leaders almost always believe, and say, that they are acting in the other person’s best interests.
Mahāvīra believes that he has vision and wisdom, and that Gotama doesn’t. Why would he want to blind someone who is already blind? Surely he would want to convey vision to him. Is that not the clear purpose of the mission: to deliver wisdom to the blind?
This means we can translate simply according to the plain meaning.
Suppose a man went to deliver testicles, but came back castrated. Or they went to deliver eyes, but came back blinded.