Unlike the original JFFS, JFFS2 treats each erase block of the flash separately. It will never write nodes which cross from one erase block to another, and will not cope if it encounters a file system with such nodes. Any node which crosses an erase block boundary will be ignored by JFFS2, and the data it contains will be lost, causing the file system to be corrupted. If there's not enough space at the end of the current erase block to write a whole node, JFFS2 will leave it blank and continue writing to a new erase block. Also, JFFS2 always starts writing at the beginning of an erase block, and does not expect to find free space in the middle. Any free space in an erase block should go all the way to the end of the erase block.
One more thing to know is that JFFS2 may use multiple physical erasable blocks as one (virtual) eraseblock if the flash partition size is large enough. This implies that if one of physical blocks within the JFFS2 eraseblock becomes bad, the whole JFFS2 eraseblock is regarded as bad, thus wasting one or more good physical eraseblocks. Albeit this is a drawback, it makes the eraseblocks accounting very simple.