Errors I came across in the eve of Compiling ns-2

"To err is Human"
These errors have visited during my compilation, I made a log here so to save my time, if they visit again...

multiple type in same declaration
This error you get when you forget to enclose ; after your class definition.

undefined reference to `Agent::Agent(packet_t)'
This error you get if you dont follow the step
"touch common/packet.cc"
mentioned in nsrthowto. This is because you have modified only .h file without modifying .cc file.
You need to tell your compiler to recompile the packet.cc explicitly, I believe the gcc compiler is clever enough to recompile the modified .cc files, but it may not track the .h files.
(Sorry for my English).