As you may know, I sell premium WordPress scripts here – http://codecanyon.net/user/ZoomIt/portfolio?ref=ZoomIt . As authors, we are not required to offer support, but I myself consider it a important part of the deal and I do offer it best as I can. 80% of support requests are related to plugin compatibilities ( the other 20% are free modifications attempts from buyers or buyers who do not read the docs ). I am listing here the most common problems:
script vs. wp_enqueue_script
Needless to say wp_enqueue_script should always be used. 90% of plugin incompabilities are because of this.
Some wise guy decided it would be better to include jQuery itself in the jQuery tools download he used – http://flowplayer.org/tools/download/index.html . Needless to say, it would crash all WP jQuery plugins. And it was also not included with wp_enqueue_script, it would crash anyway but man… Wasted me some hours, but I’m glad it’s solved.