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.
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.
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.