If someone is on a slippery slope, they are involved in a course of action that is difficult to stop and that will eventually lead to failure or trouble.
The company started down the slippery slope of believing that they knew better thanthe customer.
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slippery slope in British English
a course of action that will lead to disaster or failure
See full dictionary entry for slippery
slippery slope in American English
a course or situation regarded as easily or inevitably leading to further decline or deterioration