--- a/mex/comp_fftreal.c
+++ b/mex/comp_fftreal.c
@@ -81,7 +81,10 @@
   //        double *in, double *ro, double *io,
   //        unsigned flags);
 
-
+  /* We are violating this here:
+  You must create the plan before initializing the input, because FFTW_MEASURE overwrites the in/out arrays.
+  (Technically, FFTW_ESTIMATE does not touch your arrays, but you should always create plans first just to be sure.) 
+  */
   p = LTFAT_FFTW(plan_guru_split_dft_r2c)(1, dims,
 				   1, howmanydims,
 				   f, cout_r, cout_i, FFTW_OPTITYPE);