Here are examples of another common problem in yudim, found in cheap mzuzos and parshiyos, but it may occur in expensive ones by mistake.
The yudim don’t have a clear division between their guf (the rosh) and the regel, actualy turning them into a yud that doesn’t have a regel, which is pasul.
In extreme situations where there is definitly no diference between the rosh and regel, it is pasul and impossible to fix because shlo c'sidran, in situations where we can see some slight diference one should ask a rav if it can be fixed.
This may occur also in good mezuzos/tefilin – an example
Look carefuly at this yud, it is one piece only, plus a kotz rabenu tam, there isn't a distinction between the rosh and regel.