The sign was inappropriately placed, confusing many visitors.

inappropriately 是形容词 inappropriate 的副词形式。in- 是否定前缀，表示"不"；appropriate 意思是"合适的"。inappropriate 就是"不合适的"，加上 -ly 变成副词 inappropriately，表示"不合适地、不恰当地"。