Yes sometimes cattle guards do hurt cows.
When a cow goes over a cattle guard their hooves will go through the cattle guard which causes the cows legs to drop through the cattle guard.
Then when the heavy cow drops through the cattle guard the cow can break it's leg or legs in the process of falling through the cattle guard.
Most cows however do avoid cattle guards because they may at first try to step over the cattle guard and realize that the cattle guard will not allow them to safely cross.
Although cattle guards can stop cows from going over them sometimes cows can just go around the cattle guard if there are no other fences stopping them.
A small child could also slip through the cattle guard and break their leg.
I know that because that happened to me on my grandpas farm when I was 3 years old and decided to walk across the cattle guard and I slipped through the cattle guard.
I can remember it as I was outside on a hot summer day in the year 1991 wearing just my diaper, shirt and shoes.
I have a picture of it somewhere my mom took of me just before I fell through the cattle guard.
In those days the disposable diapers were the plastic backed crinkly sounding and thick pampers diapers that made it hard to walk somewhat.
That may have contributed to me losing my balance.
But I fell through the cattle guard and broke my right leg.
I had to go to the hospital and get a cast.