A simple example that uses two threads to show that Intel x86 does not implement sequential consistency. The example is similar to the one used by Lamport in his classic paper How to Make a Multiprocessor Computer That Correctly Executes Multiprocess Programs.

Compile with: g++ -otest.x test.cc -std=c++11 -pthread

You should never see as output both 1 and 2 printed. It is very rare, but it happens! I suggest you keep the code running for awhile, such as:

while true; do ./test.x >> log; done

then check the file for both 1 and 2 (e.g., cat log | awk 'NF {if (++n >= 2) print NR; next}; {n=0}').

This will return the line numbers where both numbers were printed in the log.