Out of Bit order Subnet Masking. It gets confused sometimes. Well, most of the time you run them concurrently.
Running a SNM of 255.255.255.240 is just fine Windows won't have a cow on this one. But 255.255.255.16... Yikes it'll screw up the XOR if the same interface has the other active SNM active also.
Only time I use OOBSNM is when I am short on available IP Ranges, and I have to get places connected using those. Most of the time I just use a 255.255.255.252 (or 4 address routing network). I rather use a variable length subnet mask than an out of bit order one. Less headache and doesn't go against the CIDR stuff(big, Big, BIG) out there.
All int all, OOBSNMs are not a truly supported option in most of the Older Routers and Switches out there. It'll confuse them as well. BE careful where you use it.