thin edge of the wedge

Some minor change or development that instigates or foreshadows something much larger or more impactful. Typically used in reference to that which will lead to an unfortunate, undesirable, or catastrophic outcome. These new driverless cars are just the thin edge of the wedge, if you ask me—pretty soon, every facet of our lives will be controlled by robots and automation! If we allow them to get a foothold in this territory, it could be the thin edge of the wedge that sees them stealing huge portions of our market share.

A minor change that begins a major development, especially an undesirable one. For example, First they asked me to postpone my vacation for a week, and then for a month; it's the thin edge of the wedge and pretty soon it'll be a year . This term alludes to the narrow wedge inserted into a log for splitting wood. [Mid-1800s]

An unimportant action that is the start of a major development, often an undesirable one. This expression alludes to the pointed metal wedge inserted into a log for splitting wood. The expression began to be used figuratively in the nineteenth century. Anthony Trollope used it several times in Dr. Thorne (1858), and it was commonplace by the end of the century. Richard Blackmore had it in The Remarkable History of Sir Thomas Upmore (1884): “My father kept calling him . . . the thin edge of the wedge, and telling dear mother . . . not to let him in.”