Detailed observations of distant Type 1a supernovae, by two independent teams, are what got astrophysicists and cosmologists salivating about 'dark energy' (and the many possible physical processes that could give rise to it). Systematic effects (including mis-identification) are, of course, the bane of astronomy, and considerable effort has been devoted to identifying, and characterising, these wrt observational results leading to DE conclusions - accurate characterisations (a.k.a. 'how can you be sure it's a distant SN1a?'), selection biases of many kinds, (cosmological) evolutionary factors (WDs of long ago were 'different' than today's), dust (grey or otherwise) in the SN galaxy, etc, etc, etc. What are your favourite papers, that slay one or more fearsome 'systematic' dragons? Which such dragon(s) do you consider still has at least some life (if only tenuous)? What's that? What are mine? Well, 'where are the hypernovae, in these distant galaxies?' is one; another is 'how good a standard candle are hypernovae?'