bibTeX and homonymy

How comes BibTeX is unable to spot homonyms?! Namely, if I quote two of my 1996 papers in the same LaTeX document, they will appear as Robert (1996a) and Robert (1996b). However, if I quote two different authors (or groups of authors) with the same surname, Martin as in the above example, who both happened to write a paper in 2014, BibTeX returns Martin (2014) and Martin (2014) in the output, hence it fails to recognise they are different authors, which is just weird! At least for author-year styles. I looked on Stack Exchange TeX forum, but the solution I found did not work with the IMS and Springer styles.

spot the LaTeX mistake

It took me forever to spot the (obvious) mistake in the following LaTeX formula

\int \exp\[ -\{ (n+\sigma^{-2})\mu^2 -2n\mu{\bar x_n} + n ({\bar x_n})^2 \}/2 \]

Can you find it?!