Seemingly patternless optlink premature termination

[pineapple via Digitalmars-d]

I'm working on my dumb library and I have run into a case where 
when I add this line to a module, I get an optlink error 
attempting to compile it:

     unittest{}

The module compiles fine without it. So do the modules which 
depend on it.

But with that line? optlink errors everywhere.

A screencap of the error box that pops up:

     http://puu.sh/tfmBK/c5068eb2fb.png

And the contents of stdout:

     checkpoint(256)
     --- errorlevel 1

I encountered a similar error right before this and eventually 
worked out that I had introduced a cyclic dependency into my 
project some time ago - strangely not rearing its head as an 
error until I made a completely unrelated addition to the tests 
in a module incidentally importing one of the modules involved in 
that dependency cycle - and addressing that issue seemed to 
resolve the optlink error. (At least until I tried to add some 
extra unit tests to one of the modules involved in that cycle.)

I am utterly failing to find a way to make a simple repro case 
from this, or to understand what could conceivably be causing 
this error to occur in a module whose dependency graph is small 
and simple.