A stepping-stone can be a stumbling block if we can't see it until after we have tripped over it.
Source: From Alt.Quotations Misc, an internet collection of quotations
-- Cullen Hightower, (?? 1923-2008), US aphorist, writer; He was a well-known quotation and quip writer from theßUnited States. He is often associated with the Americanßconservativeßpolitical movement.