The top of a iron bar disappears when there is a block above it to avoid z-fighting. However, there are some cases where it doesn't do this right.

A lower iron bar will hide it's top if there is a block above and to the side the part is on if there is a) a iron bar above it, and b) a block to the side of the bar.
This works right, otherwise you'd have stripes thru your bars.
However, you may place a block that the bar doesn't connect to.
Then, it hides the top, even though there is nothing above, creating a minor graphical glitch.