[Pazuzu]

Lots of strange things that are postulated but never seen, because the time scale is too small. for example, does a Red Giant puff off it's planetary nebula quickly (over days/weeks/years) or does it take tens, hundreds of years? Does the core gas that is puffed off ever get sucked back onto the forming white dwarf?

What about merging black holes? They might have a continuum spectrum with some intervening absorption lines.

Industrial accident? Early on in the studies of Gamma Ray Bursts, that was an actual option that was seriously discussed, as was looking down the barrel of a fusion warp drive.