Headcanon where, when Iruka and Mizuki were kids, they would train Mizuki's genjutsu skills together. They stopped for a short time when Mizuki 'accidentally' triggered a particular technique that caused Iruka to relive the Kyuubi attack, which became so severe that an ANBU member who happened to be nearby had to intervene to break him out of it.
However, this specific genjutsu-free period didn't last too long. At a time when Iruka was struggling badly with his grief, he asked Mizuki if he could cast a genjutsu where he could see his parents. He knew it would hurt, and it would make him feel worse than before but dammit, he missed them. Trying not to dwell on that, on the bright side it would count as training for both of them!
Two particular months when Iruka's pranks were hell on earth for the ANBU that had in-village patrols raised suspicion that something might be going on with him, but these two were sneaky enough to not be caught.
That was until Iruka not only wasn't breaking out of the genjutsu on his own, but Mizuki couldn't do it for him either, then he had to look for some close-by ANBU, the same from before, which appeared almost instantly when Mizuki started to panic. It was one with silver hair who was such a stuck-up but clearly a prodigy, and to their surprise, the ANBU couldn't break it either. Only when they got Kurenai in on the issue were they able to find the root of the problem, the reason they weren't able to dispel the genjutsu was that Iruka himself was strengthening it.
Mizuki got chewed up by Kurenai, who remarked that only because she specialized in genjutsu and had practice in T&I was she able to stop something like that, especially since Iruka seemed to almost perfect the basics of shinobi tactics for his subconscious to do this. Verbally bruising the younger teen's ego and the fleeting pride he felt at the chance of doing a jutsu this strong without meaning to.
When Iruka woke up in the hospital, both he and Mizuki got a tongue slash from Asuma (who was filled in by Kurenai), arguing that he told them to stop this training since the first incident and trying to not fuss over Iruka's mental well-being, then another tongue slash from the Third Hokage the same day. Ah, but Iruka was told to meet him for tea to discuss this further once he was in the right mind and discharged by the medi-nins. Iruka groaned.
Mizuki glared at the floor. Of course Iruka gets a tea meeting with the Hokage after also screwing up.
That day ended with Mizuki promising Iruka he wouldn't pull him under a genjutsu ever again, or at least nothing like that.
Part of Iruka couldn't help but feel disappointed that he now won't see his parents as lively again.