谨慎心愿违

we have a 成语：事与愿违。means getting a result against your wish. But I dont think it exactly match the English phrase. I knew this phrase from Downton Abbey and quite like it.

I think it is about the art of translation. It can be translated as: 小心你許的願望；當心願望成真 小心你想要的东西 might be ok but it sounds a little awkward. The back translation of the sentence might look like: be careful of things that you want.