Consider the output below, created with biblatex and its vanilla authoryear-icomp scheme, with the citations placed in endnotes (using the enotez package).
There isn't much that seems wrong about it. That is, until you know that the third citation command in fact points to Malinowski rather than to Knuth.
\documentclass{scrartcl}
\usepackage{blindtext,enotez}
\usepackage[style=authoryear-icomp,notetype=endonly,autocite=footnote,backend=biber]{biblatex}
\addbibresource[location=remote]{http://mirrors.ctan.org/macros/latex/contrib/biblatex/doc/examples/biblatex-examples.bib}
\let\footnote\endnote%
\begin{document}
\blindtext
\footcite{malinowski}
\blindtext
\footnote{That's also what \textcite{knuth:ct} says.}
\blindtext
\footcite{malinowski}
\printendnotes
\printbibliography
\end{document}
Notes 1 and 3 are produced using the regular \footcite command, which is sufficient since no comments are added to the citation. Note 2 though is a \footnote with a citation embedded in some text.
Nothing too special, but it seems that that's enough to confuse biblatex's ibid tracker. It doesn't seem to take note of what's happening inside note 2.
It may well be that there's something I've overlooked (in that case, what is it?). But if you people think it's something that I should contact the biblatex team about, I'll of course do that.
PS: just to clarify: this does not happen when foot- rather than endnotes are used. https://i.stack.imgur.com/Lc1f2.png

biblatexcurrently doesn't patchenotez'\endnotecommand so thatbiblatexdoesn't know it is in a footnote if you use\footnote/\endnote(if you use\footcitebiblatex does that automatically). By defaultbiblatexhas two tracking levels and it gets tangled up in those. I couldn't figure out how to patchenotezproperly, so a work-around isibidtracker=trueif you don't have citations in the text. – moewe Apr 15 '16 at 15:16