It appears that something like $$sin\alpha$$ does not work using mimetex in moodle 2.0.1+. It used to work in 1.9.5+, so the upgrade breaks many equations whose formatting is not supported in moodle 2.0.1+. The error message as shown by texdebug.php is as follows:
The shell command
"/xxx/yyy/filter/tex/mimetex.linux" -e "/xxx/moodledata/filter/tex/d71585aa298f736567a6c165a7240009.gif" – '\Large sin\alpha'
returned status = 139
File size of mimetex executable /xxx/yyy/filter/tex/mimetex.linux is 830675
The file permissions are: 100755
The md5 checksum of the file is 56bcc40de905ce92ebd7b083c76e019e
Image not found!
No text output available