Palantir Technologies (PLTR 3.46%) stock tumbled on Thursday, falling as much as 6.5%. ICE agents have been using tools supplied by Palantir Technologies to sift through tips submitted by the public in the agency's immigration enforcement operations, according to a widely circulated news report. The Department of Homeland Security (DHS) released a list of use cases for various AI tools this week. However, a wave of controversy has followed ICE in recent weeks, and negative public sentiment surrounding the federal agency is one of the things weighing on Palantir stock. Don't get me wrong: I'm a tied-in-the-wool Palantir bull, but investors should be cautious given the stock's current valuation.