Been able to reliably recreate this on the current build:
1) Take a reasonably large album (>10 images) that has not been cached
2) From the main page, click twice on the album link. (so that the second click occurs while the album is being cached but before the redirect)
You get an error similar to the following:
Warning: rename(c:\nuke\gallery2\tmp\img146.tmp,c:\nuke\gallery2\cache\1/120): File exists in c:\nuke\html\modules\gallery2\modules\core\classes\GalleryPlatform.class on line 355
Error (ERROR_PLATFORM_FAILURE): Failed renaming c:\nuke\gallery2\tmp\img146.tmp -> c:\nuke\gallery2\cache\1/120
in file c:\nuke\html\modules\gallery2\modules\imagemagick\classes\ImageMagickToolkit.class at line 268 (gallerystatus::error)
in file c:\nuke\html\modules\gallery2\modules\imagemagick\classes\ImageMagickToolkit.class at line 120 (imagemagicktoolkit::_transformimage)
in file c:\nuke\html\modules\gallery2\modules\core\classes\GalleryDerivative.class at line 522 (imagemagicktoolkit::performoperation)
in file c:\nuke\html\modules\gallery2\modules\core\classes\GalleryDerivativeImage.class at line 169 (galleryderivative::rebuildcache)
in file c:\nuke\html\modules\gallery2\modules\core\classes\GalleryDerivativeHelper.class at line 561 (galleryderivativeimage::rebuildcache)
in file c:\nuke\html\modules\gallery2\modules\core\classes\GalleryDerivativeHelper.class at line 495 (galleryderivativehelper::rebuildcache)
in file c:\nuke\html\modules\gallery2\layouts\matrix\layout.inc at line 276 (galleryderivativehelper::rebuildcacheifnotcurrent)
in file c:\nuke\html\modules\gallery2\layouts\matrix\layout.inc at line 124 (matrixlayout::_loadalbumtemplate)
in file c:\nuke\html\modules\gallery2\modules\core\ShowItem.inc at line 98 (matrixlayout::loadtemplate)
in file c:\nuke\html\modules\gallery2\modules\core\classes\GalleryView.class at line 205 (showitemview::loadtemplate)
in file c:\nuke\html\modules\gallery2\main.php at line 259 (showitemview::doloadtemplate)
in file c:\nuke\html\modules\gallery2\main.php at line 24