thin edge of the wedge
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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
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