The bug

When two dungeons generate next to each-other, there is a chance three of the chests will be placed next to each-other. This happening causes this bug to come into effect: MC-34365.
From a generation standpoint, this is very unlikely to happen. I've only seen this myself from using a custom world preset that generates a lot of dungeons.

How to reproduce

1. Create a customized world with the seed being -1270598207177083672 and the following preset:

   {"coordinateScale":43.24648,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":317.838,"depthNoiseScaleX":1.0,"depthNoiseScaleZ":1.0,"depthNoiseScaleExponent":0.01,"mainNoiseScaleX":1.0,"mainNoiseScaleY":1.0,"mainNoiseScaleZ":1.0,"baseSize":2.859155,"stretchY":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":20,"useCaves":true,"useDungeons":true,"dungeonChance":8000,"useStrongholds":false,"useVillages":true,"useMineShafts":false,"useTemples":true,"useMonuments":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":1,"riverSize":1,"dirtSize":33,"dirtCount":10,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":8,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":33,"graniteCount":10,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":10,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":10,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":17,"coalCount":20,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":9,"ironCount":20,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":9,"goldCount":2,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":8,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":8,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":1,"lapisCenterHeight":16,"lapisSpread":16}
   

2. /tp -80 16 268