MikroTik.com official announcement:
We have found a serious ACL bug in SwOS v1.7, that can damage the RB260GS. Please tell customers to avoid using ACL feature on RB260GS until SwOS v1.8 is released next week (http://www.mikrotik.com/download)
PLEASE DO NOT USE "ACL" feature with RB260GS under routerOS v1.7
PLEASE DO NOT "downgrade" 260GS to v1.6 - this will kill the switch, and require tftp reinstall to recover.
Any questions, please contact us!