Consider the following LaTeX file q.tex:

[
\documentclass{article}
\usepackage{fontspec}
\setmainfont{CMU Serif}
\usepackage[russian,american]{babel}

\begin{document}
\foreignlanguage{russian}{А}.
\end{document}
]

The letter А is a cyrillic one, U+0410. Running xelatex on it results in the message
[
This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016/Cygwin) (preloaded format=xelatex)
 restricted \write18 enabled.
entering extended mode
(./q.tex
LaTeX2e <2016/03/31>
Babel <3.9r> and hyphenation patterns for 20 language(s) loaded.
(/usr/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/share/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texmf-dist/tex/latex/fontspec/fontspec.sty
(/usr/share/texmf-dist/tex/latex/l3kernel/expl3.sty
(/usr/share/texmf-dist/tex/latex/l3kernel/expl3-code.tex)
(/usr/share/texmf-dist/tex/latex/l3kernel/l3xdvipdfmx.def))
(/usr/share/texmf-dist/tex/latex/l3packages/xparse/xparse.sty)
(/usr/share/texmf-dist/tex/latex/fontspec/fontspec-xetex.sty
(/usr/share/texmf-dist/tex/latex/base/fontenc.sty
(/usr/share/texmf-dist/tex/latex/euenc/eu1enc.def)
(/usr/share/texmf-dist/tex/latex/euenc/eu1lmr.fd))
(/usr/share/texmf-dist/tex/xelatex/xunicode/xunicode.sty
(/usr/share/texmf-dist/tex/latex/tipa/t3enc.def
(/usr/share/texmf-dist/tex/latex/euenc/eu1lmss.fd))
(/usr/share/texmf-dist/tex/latex/graphics/graphicx.sty
(/usr/share/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/share/texmf-dist/tex/latex/graphics/graphics.sty
(/usr/share/texmf-dist/tex/latex/graphics/trig.sty)
(/usr/share/texmf-dist/tex/latex/graphics-cfg/graphics.cfg)
(/usr/share/texmf-dist/tex/xelatex/xetex-def/xetex.def
(/usr/share/texmf-dist/tex/generic/oberdiek/infwarerr.sty)
(/usr/share/texmf-dist/tex/generic/oberdiek/ltxcmds.sty)))))
(/usr/share/texmf-dist/tex/latex/fontspec/fontspec.cfg)))
(/usr/share/texmf-dist/tex/generic/babel/babel.sty
(/usr/share/texmf-dist/tex/generic/babel-russian/russianb.ldf
(/usr/share/texmf-dist/tex/generic/babel/babel.def
(/usr/share/texmf-dist/tex/generic/babel/xebabel.def)))
(/usr/share/texmf-dist/tex/generic/babel-english/english.ldf)) (./q.aux)
(/usr/share/texmf-dist/tex/latex/tipa/t3cmr.fd) [1] (./q.aux)
xdvipdfmx:fatal: This font using the "seac" command for accented characters...

Output file removed.
 )
Error 256 (driver return code) generating output;
file q.pdf may not be valid.
Transcript written on q.log.
]
printed to console. The log is attached. I don't get a PDF, but I darn wish I would.

The same file does complie in an earlier TexLive version on another machine:
[
This is XeTeX, Version 3.14159265-2.6-0.99992 (TeX Live 2015/TeX Live for SUSE Linux) (preloaded format=xelatex)
 restricted \write18 enabled.
entering extended mode
(./q.tex
LaTeX2e <2015/01/01> patch level 2
Babel <3.9m> and hyphenation patterns for 79 languages loaded.
(/usr/share/texmf/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/size10.clo))
(/usr/share/texmf/tex/latex/fontspec/fontspec.sty
(/usr/share/texmf/tex/latex/l3kernel/expl3.sty
(/usr/share/texmf/tex/latex/l3kernel/expl3-code.tex)
(/usr/share/texmf/tex/latex/l3kernel/l3unicode-data.def)
(/usr/share/texmf/tex/latex/l3kernel/l3xdvipdfmx.def))
(/usr/share/texmf/tex/latex/l3packages/xparse/xparse.sty)
(/usr/share/texmf/tex/latex/fontspec/fontspec-patches.sty)
(/usr/share/texmf/tex/latex/fontspec/fontspec-xetex.sty
(/usr/share/texmf/tex/latex/base/fontenc.sty
(/usr/share/texmf/tex/latex/euenc/eu1enc.def)
(/usr/share/texmf/tex/latex/euenc/eu1lmr.fd))
(/usr/share/texmf/tex/xelatex/xunicode/xunicode.sty
(/usr/share/texmf/tex/latex/tipa/t3enc.def
(/usr/share/texmf/tex/latex/euenc/eu1lmss.fd))
(/usr/share/texmf/tex/latex/graphics/graphicx.sty
(/usr/share/texmf/tex/latex/graphics/keyval.sty)
(/usr/share/texmf/tex/latex/graphics/graphics.sty
(/usr/share/texmf/tex/latex/graphics/trig.sty)
(/usr/share/texmf/tex/latex/latexconfig/graphics.cfg)
(/usr/share/texmf/tex/xelatex/xetex-def/xetex.def))))
(/usr/share/texmf/tex/latex/fontspec/fontspec.cfg)))
(/usr/share/texmf/tex/generic/babel/babel.sty
(/usr/share/texmf/tex/generic/babel-russian/russianb.ldf
(/usr/share/texmf/tex/generic/babel/babel.def
(/usr/share/texmf/tex/generic/babel/xebabel.def)))
(/usr/share/texmf/tex/generic/babel-english/english.ldf)) (./q.aux)
(/usr/share/texmf/tex/latex/tipa/t3cmr.fd) [1] (./q.aux) )
Output written on q.pdf (1 page).
Transcript written on q.log.
]