Hmm, well, if I understood what you did, it's not a bug.

There is one thing in my todo list that I really should do (if only I had more time), and that is : hide all the translations when someone clicks on "Translate". Then people will probably understand better that they are not translating a group of sentence, but only one particular sentence.

So in your case, the sentence 339047 was a translation of an English sentence:

Can you deliver this? <-> Le importaría repartir esto?

When you added your translation, you also INDIRECTLY translated the Japanese sentence. Because initially you had:

配達してもらえませんか。<-> Can you deliver this?

And when you added your translation, here's what happened:

配達してもらえませんか。<-> Can you deliver this? <-> Le importaría repartir esto?

(See? Indirect translation.)

But when you did your search, you probably searched the Japanese sentence. And the search results only display sentences and their DIRECT translations.

So, you added a translation to the Japanese sentence, and the whole thing became linked this way:

Le importaría repartir esto? <-> 配達してもらえませんか。<-> Can you deliver this? <-> Le importaría repartir esto?

And now you have to know that when you BROWSE a sentence, we display both the direct AND the indirect translations. Which is why you will see two Spanish translations for http://tatoeba.org/eng/sentences/show/121527.
One is the direct translation. The other is the indirect translation.

Hopefully I understood properly your problem and that my explanation was somewhat clear...