PL/SQL errors can occur either during compilation, or when viewing the output from a successfully compiled PL/SQL procedure or package in a Web browser. Errors on compile always result in an error message, so you have immediate feedback. Errors when viewing the output from your code in a Web browser can result in one of two types of errors:
An explicit error message.
No error message is generated but the code does not do what it is supposed to do.
This second type of error can be more elusive to pinpoint and debug.
Compilation errors always generate an error message. A procedure/package will not compile if the correct syntax and keywords are not used, ...