Two contract clauses, can one trump the other?

[Jeremy Williams]

How does a contract work when one clause somewhat contradicts another?

For example:

In no event shall "company A" be obligated to reimburse "company B" for any cost overrun not expressly agreed to by "company A"

and

This contract is final and binding. No amendments to this contract shall be effective unless in writing and signed by both parties.

"expressly agreed to", from what I researched, can be an oral or written agreement, but this second clause states nothing is effective unless in writing. If someone from company A agrees to a change but it isn't in writing, it will be hard to prove, but the first clause makes this possible...or does the second clause make this not possible....

Also, if someone like a secretary orally agrees to a change, does the first clause hold the company responsible for her making the decision for that agreement or again, does it need to be in writing??