As stated in the documentation, Joomla's Mootools (version 1.11) needs to be unloaded when viewing SIMGallery (detailed instructions available in the documentation PDF file).

When viewing SIMGallery, right click > view source, and see if another Mootools is loaded (either Joomla's or your template's) by searching the source for "mootools". If it is, please refer to SIMGallery's documentation PDF file for information on how to disable that.

If your template uses its own Mootools, you may have to disable gzip compression in your template's configuration first (assuming your template uses gzip and provides a configuration to disable it) before viewing the page source.

If the documentation doesn't help, find out where exactly does the template or the other 3rd party extensions calls up Mootools, and wrap it with a simple condition:

if ($option != 'com_simgallery') {
 //proceed with loading Mootools 1.11
}

IMPORTANT: Please have a look at this forum.

If you are unable to modify your template, or has trouble doing so, you may purchase a custom installation for $40 SGD.

Note: read this if you have JomSocial's Latest Members module installed.