US President Donald Trump's team has ordered the termination of vital support for F-16 fighter jet jamming equipment that Ukraine has. What is important to understand is that this could deprive the Ukrainian Air Force of its most important air countermeasures.
Note: Iran still uses F-14 Tomcats.
Possibilities for implanting a logic bomb are endless, but there are still no case reports about an US-originating plane becoming remotely disabled.
(Russians would absolutely like to listen to a message which accomplishes that, meanwhile allies would definitely want ot change encryption keys on any plane which is supposed to receive that. I also don’t think that military planes will accept unencrypted messages. And their communications subsystems are separate subsystems, which can be disabled or replaced.)
Meanwhile, in a hypothetical doomsday scenario, Danish F-35s would land (or be unloaded from a container) in China, and be greeted with cheers, red banners and golden confetti. Damage to the US would far exceed anything that Denmark could do by firing something.
Note: these are scenarios which are not supposed to happen, but dramatic loss of trust among allies can actually result in that - if country A backstabs country D, there is nothing that really prevents D from betraying A to C.