Dimethyl suphide (DMS) which looks like a very short caterpillar smells like cooked cabbage or canned sweet corn. It is either a desirable flavour note or an off flavour in Pilsner style beers depending on your point of view. DMS is formed during fermentation from dimethyl sulphoxide which is formed during kilning from the precursor S-methlymethionine. The reason it is found more commonly in pilsners is that the more gentle kilning of lager malt removes less S-methylmethionine than that of ale malts. The brand Rolling Rock used to stink of the stuff. Maybe it still does, that is if it still exists? Some German pilsners are also characteristically high in DMS.
Most commodity beer brewers go to great lengths to avoid DMS in their beers. I remember being at Well Park Brewery (beer factory) in Glasgow when 4 tankers (120tonnes) of beer arrived from the Bass brewery in Birmingham. The Process Team Leader explained that they were Carling Black Label and had been sent up to blend off with beer brewed at Well Park because they had too much DMS in and the Bass brewery didn’t have any beer with low enough in DMS to dilute the DMS down below the flavour threshold.
DMS is also formed in large amounts by some bacteria and wild yeast so presence in a cask ale is often due to an infection. I know some hardened ‘lagerheads’ who didn’t object to the taste of DMS until it was pointed out to them. One of them has never forgiven me for highlighting the DMS in his favourite Canadian/UK lager brand becuase it runied his enjoyment of the beer. I managed to forgive myself in the end.