Recently a forum member found a block of beryllium copper. Luckily he was smart enough to ask about it before using it, and in the course of the ensuing thread other hazardous things that look innocent enough were mentioned. Another member suggested making a sticky thread about such things, so here it is. Feel free to add your own, after all this may save someone's health if not their life!
Beryllium copper, aka beryllium bronze, is sometimes found in junk shops. It is usually marked BeCu, and is most commonly used (at least the way most of us would find it) to make non-sparking tools