A serial number is traditionally in the format (YYYYMMDDSS) where SS the sequence number. For example, today is 1/21/2011, so the serial number in a zone file modfied today would be… 201101210.
So lets say that you pushed that zone today and when you went back to make an additional change, you accidentally added an additional number to your serials and instead of 201101211 you ended up with 2201101211.
In order to get everything back in order you will need to manually reset the zone file's serial number by setting it to the number below. Its highest number you can get using 32 bits.
To to reset your serial number, update your zone file with the serial number listed above and push it out to the slaves. Then change your zone files serial number back to the correctserial number, which will be considered to be greater than the magic number above. Then push dns again.
Check your slave servers and verify the the zone file now has the correct serial.