The end of "The Green Candle", by the end the candle's already burned out despite everyone's best efforts and Zordon explains that the only way to prevent the Green Ranger power from being stolen is for Tommy to give the power to another Ranger. So Tommy chooses Jason, who gets the Dragon Dagger and shield. Tommy demorphs. Tommy puts on a brave face but you can tell the failure to protect his Ranger powers hit him pretty hard.
It's just as hard for Jason, who blames himself for not getting the candle when he had the chance. And it haunts him for a long time until "Missing Green". Adding to this is how legitimately torn Jason was between grabbing the candle and going to save Tommy since both boys' behavior implies that in that moment, Jason was choosing between Tommy having a life worth living or having a life at all.
Believe it or not, this is actually child's play compared to the Zyuranger version, in which Tommy's counterpart Burai is outright Killed Off for Real.
At the start of "To Flea or Not to Flea", Ernie is depressed that the Youth Center is on the verge of closing down, and the Ranger teens try desperately to make enough money to keep it open. In the end, for his part in finding and taking care of a lost dog, he receives the reward: More than enough money to keep the Youth Center open and well-earned free pizza for the teens.
From the "Green No More" two-parter, the scene when Goldar appears and then shows the already dejected Tommy footage of when he was the powerful Green Ranger (some of it being from when he was still evil, thanks Goldar).
When Rito destroyed the Thunder Megazord Season Three episode "Ninja Quest", that was the last time we ever got to see the original Dinozords in action.
All the times that it appeared that the Zords had been destroyed. While Word of God has said that the "Mighty Morphin" Zords aren't sentient like their Japanese counterparts, those cries sure seemed to have a lot of emotion in them.
To make things worse, there are times where the Dinozords are referred to like they're sentient: Zordon calls Titanus an old friend in "Gung-Ho" and describes the Dragonzord as sleeping when Clone Tommy is trying to revive it in "Return of the Green Ranger" which suggests that not only are the Dinozords sentient, but that the Thunder Megazord is sentient too. Unless the Dinozords' conversion into their thunder-forms essentially kills them every time.
In "Green With Evil", each part has its moments. The most heartbreaking one would have to be when Tommy destroys the command center. Zordon, knowing well that Tommy is the Green Ranger, tries to talk him out of it, but to no avail.
Zordon: Tommy, she (Rita) has you under an evil spell, let me help. I can save you.
Tommy/Green Ranger: Worry about saving yourself, old man!!!
The destruction of the Command Center. For three years, it'd been the Power Rangers home base, and in the space of five minutes, it's blown up with the Rangers utterly powerless to stop it and seemingly with Zordon and Alpha still inside. Sure, next season, it's rebuilt as the Power Chamber and Zordon and Alpha survive, but it still means that the series is ending on one of the biggest downer moments ever.
David Yost gives the most convincing performance as Billy with his Big "NO!" seeing the Command Center burning in shambles. More than any other Ranger, the Command Center was like a second home to him. He makes his pain sound genuine.
The apparent bad blood between Austin St. John and Jason David Frank, considering Jason and Tommy's friendship, was a spot of real pain for the fans for a long time.
Happily, it seems they've buried the hatchet if them attending the premiere of the Power Rangers (2017) is any indication.
Perhaps the greatest example is the death of Thuy Trang, the actress who portrayed Trini, in September of 2001 from a horrific car accident; it left all her castmates absolutely devastated (to the point that they have difficulty speaking of it decades later) and ripped the hearts out of fans who heard the news as well, especially since Trang wasn't even 30 years old at the time of her death. Many fans, to this day, still have and make memorials and tributes dedicated to her.
This also led to some contention with the 2017 movie, as certain elements of it felt disrespectful to her memory to many long-time MMPR fans.
The fact that David Yost's orientation lead to him literally being bullied out of acting, forcing Billy's character to be unceremoniously exiled from canon, even as they tried to give him a fitting send off.
On September 7, 2019, Robert Axelrod, the voice actor for Lord Zedd and Finster, died at 70 years old from complications resulting from spinal surgery.
Cody Slaton, the actor who played Bulk as a child during the Alien Rangers saga, passed away on July 24, 2019, just over two weeks after his 36th birthday.