This may be specific to some payment processors. i am observing it with authorize.net.
Contribution page has 'credit card' and 'pay later' available as option, therefore the two payment methods are available as radio buttons.
When the page is opened, no Credit Card / Billing fields are displayed.
But, once an amount is selected, the Credit Card / Billing fields are displayed which results in users jumping down to fill those in, then getting an error telling them that 'payment method' needs to be chosen.
IF a default value of 'amount' is set, then the Credit Card and Billing field show immediately. Which does not seem correct to me.
UPDATE - this is an issue when using Dummy processor which ships with civi.