Cut corners
to do something the easy way, sacrificing quality or thoroughness
Move the goalposts
to change the rules or criteria during a process, making it harder to succeed
Paradigm shift
a fundamental change in approach or underlying assumptions in a field
Double-edged sword
something that has both advantages and disadvantages
Examples
The contractor cut corners to finish ahead of schedule, and the building was unsafe.
Every time we met their demands, they moved the goalposts.