Created attachment 33755 [details]
I am using Sans 7pt; please note that the issue appears with "medium" or "strong" hinting style; with "slight" or "none" there is no problem. Please see the attached screenshot.
On the left side you see what happens when typing the word "diffff"
notice that when passing from a single 'f' to the ligature 'ff'(U+FB00) (, the first 'f' is displaced to the left by 1 px. When adding whatever letter afterwards (in this case another 'f'), a 1px space is added; adding the forth 'f' again moves the third 'f' to the right by 1px, but does not fill the gap.
On the right side you see what happens when typing the word "dififi"
Again when passing from a single 'f' to the 'fi'(U+FB01) glyph, the first 'f' moves 1px to the left (and almost forms a "ligature" with the previous i). This time adding a letter afterwards doesn't add the 1px gap previously noticed.
Similar issues appear with the other ligatures fl, ffi and ffl