Bad translation

Perhaps when Dasein addresses itself in the way which is closest to itself [i.e. in a self-conscious utterance of "I am this being"] it always says "I am this being," and in the long run says this loudest when it is "not" this being. Dasein is in each case mine, and this is its constitution, but what if this should be the very reason why, proximally and for the most part, Dasein is not itself [italics Heidegger's]. What if the aforementioned approach, starting with the givenness of the "I" to Dasein itself, and with a rather patient self-interpretation of Dasein, should lead the existential analytic, as it were, into a pitfall? ...[Thus] the word "I" is to be understood [in Being and Time, the "existential analytic" under discussion] only in the sense of a non-committal formal indicator [italics again H's] indicating something which may perhaps reveal itself as its "opposite" in some particular phenomenal context of Being. [The following italics are mine:] In that case the "not-I" is by no means tantamount to a being which essentially lacks "I-hood," but is rather a definite kind of Being which the "I" itself possesses, such as having lost itself.
-Being and Time , Chapter IV, 151-152.

"Such as having lost itself" is the really key part. The sentence in German ends with the word "Selbstverlorenheit," or "Self-losing-ness," which does not refer to the "I"'s remaining itself and losing itself, but rather to "the Self" of Dasein (which is not merely the "I") being lost by the "I." In other words, the sentence should read like this:

In that case the "not-I" is by no means tantamount to a being which essentially lacks "I-hood," but is rather a definite kind of Being which the "I" itself possesses, such as having lost the Self.

I'll comment on the interesting ramifications of this translation in a little post after this (and don't worry, I'll complete my Heidegger posts on the ready-at-hand, too: this just came up in the meantime).