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Not sure what happened (would be easier to tell if you'd post some of the photos) - but yea, probably they just developed your slide film in C-41.
Some people develop their film in the wrong chemicals deliberately, then its called "cross processing" / "x-pro" - and this usually gives very strong contrast and a shift in colours; often blues turn to cyan very much etc...https://www.lomography.de/search/photos?order=popular&query=xpro
Ah ha.
Unbeknown to me, I had some high quality slide film developed at 'snappy snaps'as a Christmas present.!
All the pictures look as if they have been poorly developed.
They were returned like normal 35mm film, im sure they have used the wrong chemicals or something.?