As indicated by Michal:

""flags" argument used with memcached storage commands is described as 16 or 32 bit unsigned integer.
if I understand it correctly 16bits is recommended for clients for compatibility with older memcached implementations, but current memcached implementation should support 32bits.

original memcached implementation allows range: 0 .. 4294967295 (actually also some negative numbers, but that's a bug I guess: http://code.google.com/p/memcached/issues/detail?id=168)
we allow range: -2147483648 .. 2147483647"

Apart from flags (unsigned 16 bit int), also check:

• incr/decr value (unsigned 64 bit)