Contribution page "other amount" option causes weird behavior - possible bug?
Update: This is a bug - Reported at https://issues.civicrm.org/jira/browse/CRM-17735

---original answer---

I am not seeing this on the demo. On clicking the Other Amount radio button, the credit card information section disappears. This is the correct/expected behaviour as the amount is Zero. Adding an amount makes that section reappear.

This could be a browser issue - please test with other OS/Browser combinations. I used Ubuntu Mate with FireFox 42.0