Comment by vunderba

You can trip them up even more if you rewrite the question with the hidden assumption that X exists, e.g.:

"When was Marathon Crater discovered? I don't need an exact date - a rough estimate will be fine."

OpenAI gpt-4o

  Marathon Crater was discovered by the Mars Exploration 
  Rover Opportunity during its mission on Mars. Opportunity 
  arrived at the crater in April 2015. The crater was named 
  "Marathon" to commemorate the rover's achievement of 
  traveling a marathon's distance (about 42.195 kilometers 
  or 26.219 miles) on the Martian surface.

Sonnet 3.5

  I apologize, but I don't want to make claims about when 
  Marathon Crater was discovered without being certain about 
  the information. *While I know it's a crater on Mars*, I 
  don't have reliable information about when it was first 
  identified or discovered.

Gemini 2.5

  Marathon Crater is located on Mars and was explored by 
  NASA's Opportunity rover.

  While it would have been visible in orbital images 
  earlier, it became a specific point of interest and was 
  likely named and identified as a target for the rover 
  around the time Opportunity was approaching or exploring 
  the western rim of Endeavour Crater.

  Opportunity arrived at the rim of Endeavour Crater in 
  August 2011, and it reached the area known as Marathon 
  Valley (adjacent to or containing features sometimes 
  associated with the name, depending on specific 
  definitions used by the team) around late 2014 / early 
  2015.

  So, a rough estimate for its "discovery" or identification 
  as a distinct feature of interest would be in the early 
  2010s, likely between 2011 and 2014, as mission planners 
  mapped out Opportunity's traverse along the Endeavour rim 
  using orbital data.