I'm not sure how many turbo people we have on here who run water/meth injection, but I hope this info helps someone.
For those who don't know, I have a turbocharged 2006 Focus. I used to have a 2003 Mach 1, and have been on SCS since the start. Major lurker now. :D

(the story)
Time for everyone to learn from my mistake/shortsightedness and possibly save a motor. Thank fully mine was not damaged.
Last week I noticed the car was overboosting. It was set for 11psi, and running clearly uncontrolled somewhere over 15psi. I had recently tweaked with my boost controller settings, so I assumed that was the problem. Nope. There are only so many pieces that control boost, so I determined the issue was with the ball+spring in the boost controller Tee, also called the "gate".
The ball had rusted badly enough that it was stuck in place. Since the ball+spring isn't even necessary I removed it to fix the problem.

(the lesson)
The cause of this is no doubt due to my running water/meth injection AND providing the boost signal to the boost controller from the intake manifold. Dumb. I was essentially blowing some water/meth into the boost controller every time it went into boost.
I'm using a TurboSmart dual-stage boost controller, which is a bleeder style. This means that excessive boost will be bled off at the boost controller, effectively causing a small boost leak before the wastegate.

(the fix)
Option 1: Don't use that spring+ball, or at least replace the ball with one that won't rust
Option 2: Use boost signal from before WMI is sprayed, if possible, so WMI is not forced through the boost controller
Option 3: Use different style boost controller
Option 4: Don't run WMI

I'll be changing my boost controller signal from the manifold to the intercooler piping, and leaving out the spring+ball. For now I just removed the spring+ball.

I hope this saves someone a motor. In hindsight I can't believe I didn't see this coming.