Possible fix for #660.

libethcore/Exceptions.cpp

@@ -26,7 +26,11 @@ using namespace std;
 using namespace dev;
 using namespace dev::eth;
 
-#define ETH_RETURN_STRING(S) static string s_what; s_what = S; return s_what.c_str();
+#if _MSC_VER
+#define thread_local __declspec( thread )
+#endif
+
+#define ETH_RETURN_STRING(S) thread_local static string s_what; s_what = S; return s_what.c_str();
 
 const char* InvalidBlockFormat::what() const noexcept { ETH_RETURN_STRING("Invalid block format: Bad field " + toString(m_f) + " (" + toHex(m_d) + ")"); }
 const char* UncleInChain::what() const noexcept { ETH_RETURN_STRING("Uncle in block already mentioned: Uncles " + toString(m_uncles) + " (" + m_block.abridged() + ")"); }