M-w, then C-y. C-y inserts text properties that aren't on original.

[Alan Mackenzie]

In a C++ Mode buffer, I create a region containing a template-opener <,
M-w it, then C-y it.

The C-y puts a syntax-table property on the (copy of) the <, but this
property didn't exist on the original.

What _was_ on the original was a category property whose symbol had a
syntax-table property.

This is the root cause of bug 5570 (indentation messed up in C++).

Would somebody please suggest the part of Emacs I should be looking at
to debug this problem.

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).