There is a misquotation invariably cited in many discussions of common custom:
פוק חזי מאי עמא דבר
Translation: Go out and see what is commonly done.
But the context is predictably missing. This rule applies only in cases of doubt, not certainty. If you know what the truth is, all the people in the world acting otherwise does not count for anything.
Here is one sample of the rule (and all other sources match) from Eruvin 14b:
אמר ליה רבא בר רב חנן לאביי הילכתא מאי אמר ליה פוק חזי מאי עמא דבר