Learn English – Idiom for people complaining excessively

Sometimes people set a trap in order to force others to make a mistake or own up to the truth. An English idiomatic phrase that fits this type of situation is to trip someone up; if the victim falls (unwittingly) into the trap, then you have successfully caught him/her out.

set a trap (for somebody)
a clever trick that is used to catch someone or to make them do or say something that they did not intend to

catch out
to show that someone has made a mistake or is not telling the truth, especially by asking them questions He asked her casual questions to see if he could catch her out.

_{Sources: Longman Dict. of Contemporary English, Macmillan Dictionary}

You gave a hint: from nobody to somebody is a quite common expression.

There is also an idiom from Chinese culture which is the basis of your Magikarp-Gyarados example: "liyu (carp) jumps over the Dragon Gate." According to Chinese legend, when carps in the Yellow River jump over the Dragon Gate, they transform into dragons. The Dragon Gate refers to a grand waterfall at the Yellow River at Henan.

_Sources:

_{• Magikarp Power / tvtropes.org
• Asian Carp / wikipedia
• egreenway.com}