I did some optimizations on my reflection emit, and now I get the following
message from PEVerify:

Stack height at all points must be determinable in a single forward scan of
IL

I'm not doing anything especially fancy like folding common code sections
together and branching from different locations in my method body.  The
stack height is fixed for every instruction regardless of execution path.

The method initializes a BitArray field from an array of indexes to be set
(MyClass/Bits is an enum)
Here's the MSIL viewed with reflector (annotated with the stack height
before/after the instruction):
.method public specialname rtspecialname instance void .ctor([in]
MyClass/Bits[] bits) cil managed
{
     .param [1]
     .custom instance void [mscorlib]System.ParamArrayAttribute::.ctor()
     .maxstack 6
     .locals init (
           [0] [mscorlib]System.Collections.BitArray array1,
           [1] int32 num1)
0/1      L_0000: ldarg.0
1/2      L_0001: ldc.i4 16
2/2      L_0006: newobj instance void
[mscorlib]System.Collections.BitArray::.ctor(int32)
2/3      L_000b: ldarg.1
3/2      L_000c: brfalse L_002f
2/1      L_0011: stloc.0
1/2      L_0012: ldarg.1
2/2      L_0013: ldlen
2/1      L_0014: stloc.1
1/1      L_0015: br L_0028
# warning occurs here
1/2      L_001a: ldloc.1
2/3      L_001b: ldc.i4.1
3/2      L_001c: sub
2/1      L_001d: stloc.1
1/2      L_001e: ldloc.0
2/3      L_001f: ldarg.1
3/4      L_0020: ldloc.1
4/3      L_0021: ldelem.i4
3/4      L_0022: ldc.i4.1
4/1      L_0023: callvirt instance void
[mscorlib]System.Collections.BitArray::Set(int32, bool)
1/2      L_0028: ldloc.1
2/1      L_0029: brtrue L_001a
1/2      L_002e: ldloc.0
2/0      L_002f: stfld [mscorlib]System.Collections.BitArray
MyClass::internalValue
0/*      L_0034: ret
}

What's more, even if PEVerify makes a linear pass, instead of following
unconditional branches, the constraint still holds (although I think I can
knock one instruction off if I relax that.

Are unconditional forward branches forbidden?