Brillig argues why all the forbidden relationships in the series were punished.
Micah and Moth Flight: Technically, they were before both the warrior code and the medicine cat code, but perhaps StarClan had no-mates-from-other-Clans-or-medicine-cats-having-mates in mind all along, considering what happened to them: Micah died, and Moth Flight gave up their four kits to different Clans.
Ryewhisker and Cloudberry: Ryewhisker of WindClan died saving Cloudberry of RiverClan in a battle.
Featherstorm and Hal: Hal, a kittypet, was killed by his son, Raggedstar. (Who was later killed by his son, Brokenstar. Who was later killed by his mother, Yellowfang. What is wrong with this family?)
Raggedstar and Yellowfang: Brokenstar. Plus Brokenstar’s sisters who died.
Graystripe and Silverstream: Silverstream died giving birth to Stormfur and Feathertail.
Reedfeather and Fallowtail: The kits were claimed by WindClan, leading to a battle with RiverClan over them.
Oakheart and Bluestar: Bluestar had to give up her kits to RiverClan so she would become deputy instead of the bloodthirsty Thistleclaw; Mosskit died en route and she later had to face her other two kits in battle.
Crowfeather and Feathertail: Feathertail died saving Crowfeather from Sharptooth.
The Infamous Crowfeather and Leafpool Incident: Put it this way. I occasionally like to call it The Leafpool-Crowfeather-Squirrelflight-Breezepelt-Nightcloud-Lionblaze-Hollyleaf-Jayfeather-Brambleclaw Situation, and anyone who’s read Power of Three and Omen of the Stars knows what I’m talking about.
Lionpaw and Heatherpaw: This led to later antagonism between the two.
Jayfeather and Half Moon: Jayfeather had to return to the Clans, leaving Half Moon to “wait for [him] forever.”
Stormfur and Brook: The two of them were exiled from both the Tribe of Rushing Water and RiverClan before the Tribe finally accepted them back.
Alderheart and Velvet: Velvet returned to her Twolegs, leaving Alderheart forlorn during an already difficult time.
Alderheart and Needletail: Needletail died.
Hollyleaf and Fallen Leaves: Hollyleaf died in the Great Battle.
Onestar and Smoke: Darktail.
Bramblestar and Jessy: This led to difficulties with Squirrelflight. Jessy also ended up leaving the lake forever.
Needletail and Rain: Rain was killed by Darktail, casting suspicion upon Needletail.
Appledusk and Mapleshade: Their kits drowned and Appledusk rejected Mapleshade, leaving Mapleshade to vow vengeance upon him and his kin and the Clans forever. She later killed him (as well as a few other cats beforehand), dying in the process.
Beechfur and Gorsetail: Their kits went exploring and almost drowned in the tunnels; WindClan accused RiverClan (and ThunderClan) of taking them, which nearly led to a battle.
Eaglestorm and Squirrelwhisker: Eaglestorm left his own birth Clan, WindClan, to raise his son Windstorm in ThunderClan with Squirrelwhisker; the story was so hushed up that Bluestar later had no idea of Windstorm’s true parentage(but Oakheart knew, somehow?).
Yarrowleaf and Nettle: The kits were kidnapped by Sleekwhisker; Nettle threatened to kill them; Nettle and Rowanstar/claw died in the ensuing battle.
Tigerstar I and Sasha: Sasha left the Clans after realizing what Tigerstar was really like; Tadpole drowned; Feathertail was no longer friend with Sasha after realizing who her mate had been; Hawkfrost, unfortunately, took after his father.
Bristlefrost and Rootspring: The total ramifications remain to be seen, but so far, this had led to tension between them and their Clan mates, casting suspicion upon them, as well as leading to disagreements between Rootspring and his sister.
Tigerstar II and Dovewing: Actually, an article on the Warriors website called them the first interClan mates that have not ended in tragedy.
Unless you count Dovewing being named a codebreaker or their son Shadowsight being manipulated by the impostor!