[OS X TeX] Multiline math mystery

[George Gratzer]

Checking the error messages ifor my new LaTeX book,  I typeset

\[
    x_{1} + y_{1} + \left( \sum_{i < 5} \\ \binom{5}{i} + a^{2}  
\right)^{2}
\]

This is wrong; you cannot separate \left( and \left) with \\.

To my surprise, TeXShop typeset this, ignoring \\.

Any explanation?

GG
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