This can occur on Centos5 hosts

[root@host~]# ip a del 192.168.11.50/32 dev eth1
RTNETLINK answers: Cannot assign requested address

Solution:

Use the incorrect syntax instead or figure it's wanting /24

[root@host~]# ip a d 192.168.11.50 dev eth1
Warning: Executing wildcard deletion to stay compatible with old scripts.
Explicitly specify the prefix length (192.168.21.50/32) to avoid this warning.
This special behaviour is likely to disappear in further releases, fix your scripts\!