I was strip mining and had just used up my diamond pickaxe. I had just placed some cobblestone when I hit the hot key "3" where I kept my brand new pickaxe. My pickaxe was supposed to appear on the screen so I could keep on mining. But no. It had 'magically' disappeared. I looked all around, just in case I droped it instead of equiping it. No pickaxe! It was a good one too. Diamond with the best enchantents!

Note: The old and used-up pickaxe was much like the new one I tried to equip. Sometimes, I've noticed, you can pick up a used-up pickaxe even though it HAD broken - then you could use it for just a few seconds more. Now, Could it be that my new axe and the used-up one could cancel out each other some how?
Note 2: This is not the first time, I've experienced this very same bug. It has happened at least three times (funny enough right after the Microsoft takeover ... just kidding!).