Scenery setup:
Place two commandblocks with two button, so you can activate them.
Place in the first the command (say @p, hello.) Don't forget the comma after @p, it is important.
Place in the second the command (say Hello, @p.) Don't forget the point, it is important.

What I expected to happen was...:
The first commandblock give the output (reptle311, hello.).
The second commandblock give the output (Hello, reptile311.)

What actually happened was...:
The first output (@p, hello.)
The second output (Hello, @p.)

When you remove the punctuation mark after @p, it works well. The error too happens, if you use @p[...].