We all know that not all the Jews made it out of Egypt. Four-fifths of them died during Makkas Choshech because they wanted to stay in Egypt, while the ones who wanted to leave were allowed to live.
And then we had Nachum. Nachum was in a class of his own -- he didn't care whether he was in Egypt or not (which was not sufficient to kill him during this plague). But his really big sin was his sale of tallow candles to the Egyptians during the Plague of Darkness. We are forbidden to use Nachum's candles for Shabbos because of the aid he provided to the enemy with those candles.
I heard the above from a friend. I would suggest that this might explain the juxtaposition of the first two verses of parshas Beshalach. Most people know about the four-fifths from Rashi on the second verse, but that comment seems a little out of place. Why were their deaths only mentioned here, at the sea, and not earlier, where the actual plague is discussed? I think this can be explained with the above explanation: in the verse preceding this one, there is a concern that people would begin to behave like Nachum:
כִּ֣י ׀ אָמַ֣ר אֱלֹהִ֗ים פֶּֽן־יִנָּחֵ֥ם הָעָ֛ם בִּרְאֹתָ֥ם מִלְחָמָ֖ה
For Hashem said: "Perhaps the people will become like Nachum (ינחם)
when they see war"
This concern for the Nachumization of the people prompted Rashi to mention the deaths of the other four-fifths at this point, because of Nachum's unfortunate actions during that darkness.