The first mishna in the second chapter of Shabbos teaches that you are not allowed to use Nachum's fat for Shabbos candles.
ולא בשמן שריפה ולא באליה ולא בחלב נחום
[one may not light Shabbos candles with] oil set for burning [consecrated oil that has become impure], nor with [the fat from] sheeps' tails, nor with Nachum's fat
What's wrong with Nachum's fat?
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.
We have no idea what Nachum puts in his fat, as he would always continue grabbing ingredients, saying, "let's add this too!" He was so well known for this that they even began calling him "Nachum Ish Gam Zu," Nachum the "this too" guy.
Since we have no idea what's in his fat, we are concerned that there may be some things in there that are prohibited for Shabbos candle use, or just plain gross.
To expand upon Shokhet's answer, I would like to note that Nachum was making such a great business off of his candles that he did not leave Mitzraim.
ויהי בשלח פרעה את העם ולא נחם
And it was when Paroh sent the people without Nachum
Why did Nachum not leave? It's unclear whether he did so willingly - he was raking in so much money he just wouldn't leave that behind - or whether Paroh loved his products so much that he was not allowed to leave.
We can deduce, though, from the Mishnah in Shabbos, that it was the former. He did not leave by choice, and it was for that that Chazal penalized him by not allowing his candles to be used on Shabbos.
