On Stackoverflow you can see a lot of questions popping up about errors. Some users do not even know that error messages exists, others are asking about code that gives an error message but they do not understand the error message.
If the error message is common, many questions about the same kind of error appears, but it is hard to find existing Q&A about the topic.
Please add "your favorite" error message, one per answer, a short description what it means (even if it is only highlighting terms to their manual page) and a listing of existing Q&A that are of value.
This will create a list. The question is a community wiki, so you are not answering for reputation but for creating a reference list for new users. It's based on error messages. Compare with the existing Reference - What does this symbol mean in PHP? question, which works pretty well. What are common errors in PHP and what are their Solutions.
Index of Errors
Just starting, but there are already some:
Warning: Cannot modify header information - headers already sent
Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given in ... on line
Parse error: syntax error, unexpected T_XXX in YYY on line ZZZ
Fatal Error: Call to a member function ... on a non-object
