In the following code, the usage of turn breaks the alignment of the paths:
\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\path [fill=blue] (0,0) -- +(-135:5mm) -- ([turn]90:25mm) -- ([turn]90:5mm) -- cycle;
\path [fill=red] (0,0) -- +(-135:5mm) -- ([turn]90: 5mm) -- ([turn]90:5mm) -- cycle;
\end{tikzpicture}
\begin{tikzpicture}
\path [fill=blue] (0,0) -- ++(-135:5mm) -- ++ (-45:25mm) -- ++ (45:5mm) -- cycle;
\path [fill=red] (0,0) -- ++(-135:5mm) -- ++ (-45: 5mm) -- ++(45:5mm) -- cycle;
\end{tikzpicture}
\end{document}
cropped and zoomed result:

OK, that was too large.
I guess this should be tracked futher at #519 and be closed here.
Last edit: Leon Kuchenbecker 2019-02-26
Duplicate of https://sourceforge.net/p/pgf/feature-requests/96/