Subnetting errors rarely announce themselves as "bad math." More often, two devices make different decisions about whether a destination is local, a route points at the wrong boundary, or a cloud/VPN design contains two networks that cannot be unambiguously routed. These five failure modes are worth recognizing in live configurations.
1. The two hosts use different masks
Consider Host A at 192.168.10.10/24 and Host B at 192.168.11.10/16. A calculates that B is outside 192.168.10.0/24, so A sends the packet to its default gateway. B calculates that A is inside 192.168.0.0/16, so B treats A as local and tries ARP directly.
The result can be asymmetric: one direction follows a router, while the reply is sent directly or never reaches the expected gateway. Check the actual prefix on both interfaces, not just the dotted decimal mask shown in a diagram.
ip -br addr
