Users who have reached their hard quota limit cannot modify extended attributes anymore: modifying an extended attribute amounts to creating a new block, followed by freeing the old block. When the hard quota limit is reached, no more new blocks can be allocated. As soon as a single block becomes available to the user, modifying extended attributes works again.
Version 2 of the NFS protocol (NFSv2) does not implement the ACCESS remote procedure call, and instead performs some access control decisions at the client machine, based on the file mode permission bits. It grants the user access to cached files if it thinks access is granted be the file mode permission bits. This logic is no longer correct if access control lists are in effect; both false positives and denials might result.
Write access is not affected, as all writes are authenticated on the server in all cases.