Consider following command sequence:

127.0.0.1:6379> flushall
OK
127.0.0.1:6379> BITFIELD mystring SET i8 0 10 SET i8 64 10 INCRBY i8 10 99900
1) (integer) 0
2) (integer) 0
3) (integer) 60
127.0.0.1:6379> flushall
OK
127.0.0.1:6379> BITFIELD mystring SET i8 0 10 SET i8 64 10 INCRBY i8 10 99900
1) (integer) 0
2) (integer) 50
3) (integer) 60
127.0.0.1:6379> flushall
OK
127.0.0.1:6379> BITFIELD mystring SET i8 0 10 SET i8 64 10 INCRBY i8 10 99900
1) (integer) 0
2) (integer) 8
3) (integer) 60
127.0.0.1:6379> 

If you look at the output of the second subcommand you see the output is 2) (integer) 0 on the first call and 2) (integer) 50 on the second call.

A FLUSHALL and restart after the BITFIELD command was run the first time produces consistent results.

Server

redis_version:3.2.4
redis_git_sha1:070d0471
redis_git_dirty:0
redis_build_id:2a16b085f0aaf046
redis_mode:standalone
os:Darwin 15.6.0 x86_64
arch_bits:64
multiplexing_api:kqueue
gcc_version:4.2.1
process_id:69460
run_id:9b4e088b2a092f3e973450524fdca2be6b6ea437
tcp_port:6379
uptime_in_seconds:335
uptime_in_days:0
hz:10
lru_clock:645669
executable:/Users/mpaluch/git/lettuce/redis-server
config_file: