What is one real-life example of the legislative branch checking the executive branch?
The legislative branch can check the executive branch in several ways. One is to impeach and remove from office a President who has committed "high crimes and misdemeanors" while in office. The House of Representatives has impeached two Presidents, Andrew Johnson and Bill Clinton, but neither was removed from office by the Senate. Just two weeks ago, some Democratic members of the House introduced articles of impeachment against current President Donald Trump, but these articles were not approved. Another, more routine check on the executive is the power to refuse to approve Presidential nominees to the federal courts and to positions in the federal bureaucracy. Last week, a Senate committee questioned several of President Trump's nominees to federal district court, and one, Matthew Peterson, chose to withdraw after he struggled to answer some fairly mundane legal questions posed by the committee. So while the Presidency is still extraordinarily powerful, the Congress still retains a number of powers to check those of the executive.
The legislative branch is constantly fighting the executive branch. The founders of the Constitution placed two very important checks within it. The first is the power of the purse, meaning the Congress can defund parts or all of the federal budget limiting the power of the president. Another major check on the executive is the Congressional approval process required for the appointment of federal judges. in addition to these Congress can override a presidential veto with a super majority vote, although that seems impossible given the current political climate.
Currently the legislative is fighting the executive over the funding of planned parenthood. Congress has voted to defund it, but the president has vowed to veto any spending measure which does so. The last Supreme Courtt nominee to be rejected by the Senate was Robert Bork, nominated by Reagan in 1987. This is an example of the checks the legislative has on executive priviledge of appointments.