Certain mobs will not be able to "see" redstone repeaters when pathfinding, and will try to walk towards a point that they cannot access due to the repeater. This is most noticeable when aggravated on a player or other entity, such as a snow or iron golem. If you replace the repeater with another block, such as a full block, the mob will not be aggravated on the player or other entity. Mobs that are not affected by this are the ones with the dimensions that mean they cannot be put in a box where repeaters will block their movement, such as creepers and wither skeletons.

Steps to reproduce (for 2 block tall mobs, change accordingly to other mob dimensions:
1: Build two pillars 3 blocks high 1 block apart and connect them with a block in-between them at the top.
2: Build a small fenced off area connected to the arch on one side.
3: In the middle of the arch, place a redstone repeater
4: Spawn a hostile mob, say, a zombie inside the fenced off area.
5: Go to the opposite side of the arch and set your gamemode to survival
6: (x)The zombie will try to attack you, even if its path is blocked by the repeater

Expected result:
The mob would not be aggravated towards the player

Observed result:
The mob is aggravated on the player

Note:
To reproduce for mobs like endermen, make the arch 1 block taller