UPDATE: Found this get hit through our support channels several times last week. As it turns out, depending on the missing content placeholder (or other pieces), the cause of the error may be different than just creating new list items; like I previously stated. Therefore, I "genericized" my post to be more applicable to a wider-range of situations. Same steps apply to find what's causing the issue, though.
After changing to a customized master page, creating new items for several lists, along with several other operations, no longer works and result in an error message.
"An unexpected error has occurred."
There may be several causes. One, is that the custom master page is missing necessary content place holders.
Method 1: Identify the missing content place holders by modifying the web.config.
Method 2: Find the missing content place holders manually.
NOTE: In SharePoint Designer 2007, go to View | Toolbars | Master page to bring up the master page toolbar. This may also help in identifying place holders currently used on the customized master page.
I hope this helps some of you SharePoint branders out there running into similar problems. Certainly, there are few things more frustrating then finding out your beautiful and slick-looking master page is actually causing SharePoint to fail. With that said, I am interested in learning what kind of problems you've ran into with customized master page files with SharePoint, and how you resolved the issues.