Not only criminals often use GPS jammer to prevent GPS reception. They steal valuables, hide delivery drivers from their employers, and do illegal business from their spouses. Various security agencies have taken "electronic countermeasures", particularly to protect world leaders. This is an example of a commercially available product.
At the G20 summit in Hamburg this month we did not hear any reports of GPS reception problems, but the German air traffic control is of course hoping for a report.
"Due to the G20 summit, GPS disruptions at Hamburg TMA are to be expected" (Terminal maneuvering area)
It is uncommon to issue warnings about such specific events.
The German authorities announced in the Notice to Pilots (NOTAM) that from July 6th to 9th:
In another communication to navigation, they stated that the G20 summit could cause GPS interference during the same period, so that the approach procedure with GPS was temporarily suspended.
It's not uncommon to see a navigation notice indicating that GPS may be unreliable. We just did a quick review and found that there are 42 navigational cues in the United States that indicate GPS reliability.
Internationally, the Philippines, Turkey, Egypt and North Korea have issued regular warnings about aircraft with unreliable GPS.