Here's an sampling of various goofy bugs we've had to deal with in the CLR Debugging services over the past. I mention these so that you can consider whether most of the hot "silver-bullet" testing / software development technique would ever catch bugs like this. Some of these are watered down from the original problem to make it easier to understand. (Yes, we've fixed all of these)
The CLR crashed if you stop at an assembly instruction in a generic method in managed code at a non-sequence point with the IL stack depth =2 and then take a callstack.
Trying to call Debugger.Log() with exactly 150 characters crashed.
Lots and lots and lots of race conditions. For example, thread 1 is trying to setup a funceval while thread 2 happens to start a garbage collection (which is very random event), which corrupts an object referenced in the func-eval parameters, which caused the CLR to crash 2 hours later.
On checked-only build of the product, Japanese-only OS, step-in became a step-over when you try to step-in to a method going through an interface call where the interface has a particular attribute on it, and where the caller is an optimized ngen dll and the target is a non-optimized, non-ngenned, unjitted method.
We weren't handling getting native debug events from Suspend threads. (Naively, you'd expect a suspended thread wouldn't generate debug events). Turns out the thread dispatched the native debug event to the debugger; and then we suspended it. Dispatched native debug events will still be retrieved from WaitForDebugEvent. I briefly alluded to that here. (I could do a whole post on goofy issues with the native debug API).
Debugging would hang if we got a native breakpoint debug event (0x80000003) from an IP at which there was no int3 instruction. (We didn't think this was possible).
On win98 only, doing an async-break that just happens to be in the fraction of a second when the debuggee is unloading an appdomain, and then setting up a func-eval to evaluate a method from the to-be-unloaded appdomain on another thread, and then continuining may crash.
If you attached during a module load, detach, and then reattach, we missed a module load notification on the reattach (which eventually leads to a crash). (Related to issue here).
CLR crashed from races when doing an "detach from debuggee" while simultaneously killing the debuggee from task-manager. Timing is such that it only reproes when the debugger is not being debugged.
Perhaps you could claim bugs like the ones above are too much of a corner case. The problem is when you get millions of people using your product, enough people hit the corner cases that you still need to think about them.