They can sure swim around, no doubt, I find a good cure for that is to keep them in context.
For example, regarding whether Christ's body is actually present in the Eucharist:
“For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body.”
This is very clear about Christ's bodily presence in the Eucharist, and should help solve any doubts about the type of "real" presence the Scriptures mean.
Bear ICOR 11:21 and the surrounding verses in mind. Paul is expressing outrage or at least concern over the way at what purports to be but fails to be "the Lord's supper" each one eats his own food, so one is hungry and another drunk. AND then AFTER the verse you cite, Paul appears to sum up with a solution to the courtesy and inequity problem: namely wait for one another and don't use this meal to satisfy hunger.
Consequently, the argument goes, (advocatus diaboli here) "discerning the body" just means discerning that the congregation is united as members of one body in Christ and so one's comportment at the mass should show that one sees that.