C++

• Declaring a static member variable, but not reserving space for it in the implementation of the class. This will result in an error when linking:
  /tmp/cca001861.o(.text+0x4): undefined reference to `A::Aint'.

• No ; at the end of a class definition. Can result in lots of compile errors.

• Forgetting to write the classname in front of the function name in the implementation of that member function, for example int insert_element() { .. } instead of
  int set::insert_element() { .. }.

• Defining a member function like
  int foo() const; and implementing it as
  int bar::foo() { ... }. The const should also be added to the function-header of the implementation.