There's a well known midrash in this week's parsha. It interprets the verse 've-lo karav zeh el zek kol ha-layla' (Shem. 14:20) to mean that the attending angels wished to sing before God at that moment, but He did not allow it, saying "My handiwork is drowning in the sea, yet you sing?!"
In truth, this midrash is presented in a number of places with subtly different meanings.
Perhaps the best-known appearance is in the Gemara in Megillah (10b). There, it is used as a prooftext for the assertion that God does not rejoice at the downfall of the wicked. Clearly, the meaning of the midrash is that the angels wished to sing when the Egyptians drowned, much as the Israelites themselves sang. God responded by asking, rhetorically, if it was appropriate to sing while His handiwork, the Egyptians, drowned. This begs the following question: why, then, were the Israelites allowed to sing? The standard answer is that they had experienced salvation themselves, had just been freed from Egyptian bondage, and thus had reason to celebrate. The angels, however, had not themselves experienced salvation, and it would have been inappropriate for them to rejoice when one nation is saved at the expense of another.
Another possible answer depends on a variant text of the midrash. The way it's recorded in Yalkut Shimoni, God's rhetorical question ends with the words 'before Me'. In other words, the issue was not the singing per se, but singing before God. He did not want to participate in a celebration of the downfall of men, no matter how wicked. The Israelites sang, but not 'before God'.
A second, different appearance of this midrash is in Eicha Rabbah (petichta 24), amongst other places. It lists three times when the angels wished to sing, but God did not allow it: during the flood, at the sea, and during the destruction of the First Temple. The same prooftext of 've-lo karav...' is used to support the second assertion. Here, it seems clear that the meaning of the midrash is altogether different. It is not discussing any 'extra' song that the angels wished to sing, rather the songs that the angels sing before God every day. Because of the dire situation of the Israelites during the night before the splitting of the sea, God did not wish to be sung to. Indeed, this interpretation fits better with the prooftext, which speaks about the night before the miracle. In this version, God's rhetorical question does not appear.
The 'moral' of each version is quite different. The moral of the first is that God and, presumably we should follow suit, would never view the destruction of humanity, no matter how evil, as a cause for celebration. It might be necessary, but it is with a heavy heart. The second version simply states that God is in no mood to listen to the choir of angels while His people sit in distress; it is an issue of solidarity with those in trouble, not of sensitivity toward the wicked.
My whole life, I only knew of the first version. I am curious if there are any sources which conflate the two versions or if the two versions are somehow related. It seems that they are not.