1. Poor interpretation: "distribution of compact support" brings up nothing very useful in DDG -- a bunch of linkfarms, it looks like -- but in Google the third result for me is a relevant .pdf from Oregon State.
2. Users attempting to ad-hoc convert math notation into ascii: Google understands "l 2 space", but DDG thinks I mean "l2space" which is some sort of gallery in I don't know where.
I think this actually homes in on one of the bigger issues with setting up a competing search engine, you have to be equal to or better than the incumbent all the time, and that is really hard.
The use of familiar everyday terms for technical mathematical objects makes this difficult ("net", "space", "compact", etc.). Since most pages are either all about math or have nothing to do with math, I wonder if there's an easy way to tag the former pages, which would then be used as an advanced filter available when searching. ("Is this a math search?")