HEY and CHET, VAV and ZAYIN - Letters Making a Big Difference

Now that I have the possibility to compare my original  notes from 2002 and 2003 to the death register, I discover a pitfall:
The year of death on the gravestones are written in Hebrew letters with each letter having a number value.


VAV is 6. ZAYIN is 7. 
They have some similarity in shape.
   ז           ו  
When you inscribe the letters/numbers is stone, years pass, you have moss and erosion, then I see that I have sometimes  read VAV as ZAYIN and ZAYIN as VAV.
F.ex. I had noted that the person died in the year 5676, but it was actually 5677.


HEY is 5. CHET is 8.
ח         ה
Here too, I find that I now and then have read HEY as CHET and CHET as HEY.
F.ex. I had noted that a person died in 5678, but he actually died in 5675.