Walking under a Ladder
Avoiding ladders used to be linked to the story of the Crucifixion: one kept away from the instrument that had taken Jesus down from the cross,or else one would suffer bad luck. This is highly suspect reasoning,since no ladder is mentioned in the Bible. The superstition has also been linked to the fear of entering the Holy Trinity's emblematic triangle,formed by ladder,wall and ground. Maybe the simplest reasoning for not walking under a ladder is that in going around it you are less likely to be rained on paint or hammers.
Notes:
Crucifixion 耶酥被钉在十字架上
suspect 可疑的
the Holy Trinity （圣父、圣子、圣灵）三位一体
emblematic 象征的
triangle 三角形
hammer 锤子