The file needn't be complicated to cause this.
Type in 16 spaces, then hit Enter.
The first line will be spaces, the next tabs.
Or even more strangely, copy-paste will replace spaces with tabs.
Type in Enter, then 16 spaces.
ctrl->a, ctrl->c, deselect, ctrl->v
Second line has 16 spaces, third has 4 tabs.
The file that was actually causing this problem was a Ruby script with inline quotation. I copy-pasted it over from elsewhere and was surprised to see my program suddenly failing.
Running Intype 0.9.2 in Windows 7