The dangers of asynchronous design...

I have been pushing my own limits in some of my most recent designs. I have been synchronizing less, and I have been building delay dependent logic. This is interesting and challenging at the same time. Building logic that is low-power, efficient, and high speed is not so simple. Here is an example of mistake I caught myself making:

The second ff was gonna be metastable. And the output from the second ff was gonna be synchronized externally. I didn't care what value the second ff got, just that the third ff and the external synchronization flops  would get the same value. What I had forgotten to take into account was how metastability works. I forgot that the second ff can go high during metastability even though it may start and settle low.

 Luckily I caught this soon after I wrote it.