The following program:
//------------------------------------

#include<iostream>
#include<list>

class sample
{
public:
    double criterium;
    sample(double _criterium):criterium(_criterium) {};
}; 


int main(){
  
  std::list<sample > samples;
  samples.push_back(sample(1.));

  double tmp=0.;

  std::list<sample >::iterator i1;
  i1=samples.begin();	
  tmp=(*i1).criterium; 
  // !!!!!!!!!!!!!!!!!!!!!
  // THIS += operation fails
  (*i1).criterium+=2.; 
  //!!!!!!!!!!!!!!!!!!!!!!!!

  std::cout<<"tmp "<<tmp<<" (*i1).criterium "<<(*i1).criterium <<std::endl; 
  
  return 0;
}
#----------------

produces the output

tmp 1 (*i1).criterium 1

when compiled with '-O2' or '-O3'
The right output would be 

tmp 1 (*i1).criterium 3

which occurred if compiled with -O1 or without optimization at all. 
The bug was found with the two versions 4.1.2 and 4.1.3 but NOT 
with 3.4.6 and 4.2.1.

In the following you find the output of the compilation with
 g++  -v -save-temps  -O2 

================================================
System 1:
========

Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --with-tune=i686 --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)
 /usr/lib/gcc/i486-linux-gnu/4.1.2/cc1plus -E -quiet -v -D_GNU_SOURCE tst_short.cpp -mtune=i686 -fworking-directory -O2 -fpch-preprocess -o tst_short.ii
ignoring nonexistent directory "/usr/local/include/i486-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../i486-linux-gnu/include"
ignoring nonexistent directory "/usr/include/i486-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2
 /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/i486-linux-gnu
 /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/backward
 /usr/local/include
 /usr/lib/gcc/i486-linux-gnu/4.1.2/include
 /usr/include
End of search list.
 /usr/lib/gcc/i486-linux-gnu/4.1.2/cc1plus -fpreprocessed tst_short.ii -quiet -dumpbase tst_short.cpp -mtune=i686 -auxbase tst_short -g -O2 -version -o tst_short.s
GNU C++ version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21) (i486-linux-gnu)
        compiled by GNU C version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21).
GGC heuristics: --param ggc-min-expand=99 --param ggc-min-heapsize=129483
Compiler executable checksum: 183d42a838ed2b7313bffcb8f2f2fda7
 as -V -Qy -o tst_short.o tst_short.s
GNU assembler version 2.17 (i486-linux-gnu) using BFD version 2.17 Debian GNU/Linux
 /usr/lib/gcc/i486-linux-gnu/4.1.2/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o tst_short_O2 /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crt1.o /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.1.2/crtbegin.o -L/usr/lib/gcc/i486-linux-gnu/4.1.2 -L/usr/lib/gcc/i486-linux-gnu/4.1.2 -L/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib -L/lib/../lib -L/usr/lib/../lib tst_short.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/i486-linux-gnu/4.1.2/crtend.o /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crtn.o



================================================
System 2:

Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)
 /usr/lib/gcc/i486-linux-gnu/4.1.3/cc1plus -E -quiet -v -D_GNU_SOURCE tst_short.cpp -mtune=generic -fpch-preprocess -o tst_short.ii
ignoring nonexistent directory "/usr/local/include/i486-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../i486-linux-gnu/include"
ignoring nonexistent directory "/usr/include/i486-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.1.3
 /usr/include/c++/4.1.3/i486-linux-gnu
 /usr/include/c++/4.1.3/backward
 /usr/local/include
 /usr/lib/gcc/i486-linux-gnu/4.1.3/include
 /usr/include
End of search list.
 /usr/lib/gcc/i486-linux-gnu/4.1.3/cc1plus -fpreprocessed tst_short.ii -quiet -dumpbase tst_short.cpp -mtune=generic -auxbase tst_short -version -fstack-protector -fstack-protector -o tst_short.s
GNU C++ version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2) (i486-linux-gnu)
        compiled by GNU C version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2).
GGC heuristics: --param ggc-min-expand=64 --param ggc-min-heapsize=64316
Compiler executable checksum: 3cc47be363985179cfafdceddd0e8f5d
 as --traditional-format -V -Qy -o tst_short.o tst_short.s
GNU assembler version 2.18 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.18
 /usr/lib/gcc/i486-linux-gnu/4.1.3/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both -dynamic-linker /lib/ld-linux.so.2 -o tst_short /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crt1.o /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.1.3/crtbegin.o -L/usr/lib/gcc/i486-linux-gnu/4.1.3 -L/usr/lib/gcc/i486-linux-gnu/4.1.3 -L/usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib -L/lib/../lib -L/usr/lib/../lib tst_short.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/i486-linux-gnu/4.1.3/crtend.o /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crtn.o