Recently has installed the RouterOS v4.6 on my RB750. After reboot I noticed, that one of my computers is unable to connect to RB750 (neither with winbox, nor with telnet, pinging doesn't work too - the MAC-address of RB750 doesn't resolve with ARP). After doing some experiments I stated, that any computer having 00, 01, 02, 03 or 04 in the 2nd octet of its MAC-address (00:13:76:28:00:06, for example) is affected. Changing this address to 00:13:76:28:05:06 makes it work. Writing the 00:13:76:28:00:06 address to another machine leads to similar connection loss. Changing any other nibble wasn't helpful. No MAC-filtering was made - the device is just reset to default configuration.
This was observed on RB750 with RouterOS 4.6 only - downgrading to 4.5 solves the problem. Another boards (RB600, RB411, RB433) wasn't known to produce such a bug.