You probably right about "weird" fonts, I checked and saw "dingobats" font installed, those icons look pretty much like that font. Probably Opera not scaling it properly.
Just tried to grey out messages behind blue ones in different orders and looks like I need click just one "right" message for it to turn grey. When I tried it earlier it appeared like I need click all of them because most likely "right" one was the last one I clicked
I see now in screenshot that gray hand have one bottom message grayed - I sure this is message it belongs to.
Mystery is solved