Two other metaphors come to mind:
the genie I let out of the bottle, the Pandora's box I opened

Eine Quelle gibt es eigentlich schon: Goethes Zauberlehrling.

"Die ich rief, die Geister, werd ich nun nicht los"
(Aus dem Gedächtnis zitiert, daher möglicherweise nicht ganz im exakten Wortlaut).

In that vein, Wiki had this to say:

Der Zauberlehrling is extremely well-known in the German-speaking world. The lines in which the apprentice implores the returning sorcerer to help him with the mess he has created have turned into a cliché, especially the line Die Geister, die ich rief ("The spirits that I called"), a garbled version of one of Goethe's lines, which is often used to describe a situation where somebody summons help or uses allies that he cannot control, especially in politics.

A literal translation is "to call up the spirits", often used with the object "of the dead/depated". So you could say "beware of the spirits that you call". In English literature Dr Faustus by Christoper Marlowe agreed to call up the spirits to give himself more power.

peinlich...der bezug zu goethe war mir so gar nicht bewusst...ich musste damals ein anderes gedicht auswendig lernen *rot anlauf* "Der Handschuh"

danke für die hilfe!!!

Also the German title of the film with Bill Murray,-Scrooged

Und hier das deutsche Original des Gedichtes neben der (gebräuchlichen) englischen Übersetzung: "the spirits that I called"

A more current English expression that is equivalent is "beware of what you ask/wish for" (subtext: because you just might get it).

This might work well to avoid any heavy literary references in your translation, with all the respect I owe to Goethe.

@bunnen I was thinking of that too, and agree with your reasoning. I remember it as "beware of what you wish for"

I am not at all satified with the translation "beware of what you ask/wish for". The spirits that one calles don't necessarily come by request or wishing, but also by poor desicions or bad/foul actions.

eg: If I chose to accomplish a goal by ill actions and those action start to backfire, then I didn't request or ask for anything. It's more of a summoning process of evil spirits by those reckless actions.

I mean I would go along with the sentence: "You asked for it!" or "You had it coming!". But right now I am looking for a sentence, that one would use as a warning before it already started to backfire.

"For whan a man hath over-greet a wit,
Ful oft hym happeth to mysusen it."
(Chaucer)