Tested a bunch of times. Works most of the time. Get a zombie to follow you and walk next to fence gates (tested with them in a straight line, and in a diagonal zig zag type of way). When he reaches you the zombie will cause you damage, but then it will just stand there after the one hit. He will act as if you aren't anywhere near him. Sometimes he will continue to hit you every 30 seconds or so, but it always is a hit, then wait. I looked for a duplicate, didnt find one so i hope this isn't a duplicate. Just realised I never tested with netherbrick fence though, I used normal fences UPDATE: After further testing I have realised it mainly works when facing south, and rarely when facing North. So far I could not replicate the bug with East & West.