failing to properly evaluate the (relative) importance (of); putting the cart before the horse; mistaking the cause for the end; mistaking the insignificant for the essential; getting one's priorities backwards
reversing the order of (relative) importance (of); putting the cart before the horse; the tables being turned; mistaking the insignificant for the essential; mistaking the means for the end
reversing the order of (relative) importance (of); putting the cart before the horse; the tables being turned; mistaking the insignificant for the essential; mistaking the means for the end
watch your step; slippery floor