કાગળની હાંડી ચુલે ચડે નહીં.

kāgaḷanī hāṁḍī culē caḍē nahīṁ.

Literally: A paper pot cannot be put on the stove.
What it means: A weak or flimsy plan cannot withstand the pressure of reality.

Also said as

ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1923) ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1921) · read from the scan by machine — the Gujarati itself is unchecked · English is machine-made and nobody has checked it yet

Heard it differently, or know we've got the sense wrong? Tell us — that is exactly the kind of correction this needs.