Just do the following:
<pre>
std::vector<cv::Mat> foo; foot;
cv::buildPyramid(cv::Mat(), foo, 4);
</pre>

I tracked it down to @pyrDown_@: I believe the "@limit@" variable in there can be set to 0 (for an empty matrix), thus leading to @for( ; x < limit; x++ )@ looping forever.

I am not sure of what the proper behavior is: should there be an assert ? or should it return an empty matrix ? Just let me know and I'll patch it.