Rendering- Bad frames?

[TridentMarketing]

Hello. I rendered a sequence animation and got some random frames with the background visible trough solid meshes. anyone can tell me why this happens? (Most frames came out good)
cheers.

[TridentMarketing]

A few more examples. I still do not know why this happens. any info is appreciated. At the moment I have either to photoshop those bad frames or re-render them.