Given the code:

----------
module bug;

struct S { this ( this ) {} }
struct Z { const( S )[ int ] broken; }
void main () {
	foreach ( i,x ; Z().broken ) {}
}
----------

The compiler correctly errors, but does not provide enough information in the error message:
> bug.d(3): Error: function bug.S.__postblit () is not callable using argument types ()
> Error: *&this is not mutable
> Error: this is not mutable
> Error: this is not mutable

While correct, it does not reference the line with the foreach, which is what triggers the problematic postblit.  The fix is obvious: redefine postblit as this(const this); but it isn't always obvious when looking at hundreds of lines of code.  I know, because I just dealt with it myself, and spent at least an hour figuring out what was triggering it so I could decide whether the fix was appropriate.

Maybe all postblits should be this(const this) in the first place?