According to some news sources, an incident occurred on April 11th, 2014, between a Russian SU-24 fighter jet (some articles refer to it as a "tactical bomber") and USS Donald Cook (a guided missile destroyer) which at the time was doing "patrol work" on the western Black Sea.
This article here explains the events as follows:
Meanwhile, the Russian Su-24 that buzzed the USS Donald Cook carried neither bombs nor missiles but only a basket mounted under the fuselage, which, according to the Russian newspaper Rossiyskaya Gazeta 2, contained a Russian electronic warfare device called Khibiny.
As the Russian jet approached the US vessel, the electronic device disabled all radars, control circuits, systems, information transmission, etc. on board the US destroyer. In other words, the all-powerful Aegis system, now hooked up - or about to be - with the defense systems installed on NATO’s most modern ships was shut down, as turning off the TV set with the remote control.
The Russian Su-24 then simulated a missile attack against the USS Donald Cook, which was left literally deaf and blind. As if carrying out a training exercise, the Russian aircraft - unarmed - repeated the same maneuver 12 times before flying away.
Above article further states that 27 US Navy sailors requested to be relieved from active duty as a result:
Since that incident, which the Atlanticist media have carefully covered up despite the widespread reactions sparked among defense industry experts, no US ship has ever approached Russian territorial waters again.
According to some specialized media, 27 sailors from the USS Donald Cook requested to be relieved from active service.
I am not depending purely on above article for this story, however. An issue here is that I have not been able to find what I would consider "mainstream" English language news sources that would corroborate above statements about the event and the 27 sailors.
A reason for this lack of information in English language media is likely as explained in this article at UK's Daily Mail, which states:
The official was not authorized to discuss the incident publicly so spoke on condition of anonymity.
According to some news sources, the Russian fighter came as close as 10 feet from USS Donald Cook (implying that the harassment was more based on flying too close to the navy ship), while others, such as above Daily Mail article, state that it was as close as 1000 yards (3000 feet).
I have seen some references to Russian electronic warfare around Ukraine in mainstream European media. One example is this article (in Swedish) in the Swedish daily Svenska Dagbladet which refers to statements made by US Lt. Gen. Frederick ("Ben") Hodges to the US-based Defense News.
Above article in the Swedish daily is probably referring to this article in Defense News, which says the following about Russian electronic warfare:
Ukrainian forces have grappled with formidable Russian electronic warfare capabilities that analysts say would prove withering even to the US ground forces. The US Army has also jammed insurgent communications from the air and ground on a limited basis, and it is developing a powerful arsenal of jamming systems, but these are not expected until 2023.
"Our soldiers are doing the training with the Ukrainians and we've learned a lot from the Ukrainians," said Lt. Gen. Ben Hodges. "A third of the [Ukrainian] soldiers have served in the ... combat zone, and no Americans have been under Russian artillery or rocket fire, or significant Russian electronic warfare, jamming or collecting — and these Ukrainians have. It's interesting to hear what they have learned."
Hodges acknowledged that US troops are learning from Ukrainians about Russia's jamming capability, its ranges, types and the ways it has been employed. He has previously described the quality and sophistication of Russian electronic warfare as "eye-watering."
Still these quotations do not say much about what (if anything) happened on April 11th, 2014 to cause 27 sailors to ask to be relieved from active duty.