Would require a very, very specific instance to even consider, so I don't really think it's worth learning. It'll likely never come up.
I would, however, like to know why it happens. I don't recall it being this way early 1.7, instead being very recent. Seems strange, though, because I've seen it happen on a few different servers, so it'd have to be an update to a common plugin like NCP, or a client side issue with different versions...
Can anyone confirm if this happens in vanilla multiplayer? (Fishing rod doesn't work in vanilla btw.) If so, in what version? (1.7.?, 1.8.?, a combination?) I suspect it may have to do with some way 1.8 (or another) client version handles multiple simultaneous damage sources. (Can't hit someone with a sword a second time until the first tick of it wears off, but you can hit someone with a sword after a fire damage tick - they are separate with different cooldowns. Might be that the code now works where if any cooldown is in place, it gives immunity to knockback.) If this is the case, then 1.8 may be advantageous, (or the opposite.) but it really depends on how it works, and potentially, what end of the sword/fishing rod you're on.