After the fourth flight of my most recent flying session, I could not separate the XT60 connectors.  That brought a quick end to my flying fun for the day.
When I got home I tried every thing I could think of to separate the connectors and finally resorted to wedging a sharp chisel at the part line and carefully rocking it to pry them apart.
Upon close inspection, I could see nothing on the male or female side of the XT60 connectors that would indicate that there was anything different than the other 4 batteries that I brought. 

Any reason that it would not be advisable to employ a skim of dry films lube like Neo-Film 2 and Dri-Slide or a very light skim of conductive grease like "NO-OX-ID" on the pins inside the XT60 connectors to keep this from ever happening again?  A search of RCGroups and Google shows folks using several types of products. 

https://neolube.global/dry-film-lubricant/
https://www.uniquetek.com/product/T1247
https://www.sanchem.com/electrical-contact-lubricant.html#:~:text=NO%2DOX%2DID%20%22A,free%20from%20rust%20and%20corrosion.

Is it the shells or the pins that got stuck?  If it was the shell a little silicone grease might help.  Part of the problem could be that the shell of the connector got deformed when soldering the wires.  When soldering connectors always have a mating connector plugged in.

Be careful conductive grease could cause a short.   

Hi John,  I've used these packs for several years now.  As you mentioned, I did solder the XT60 connectors with the mated connector plugged in. 
These batteries have had many cycles.  This difficulty with removal is a new, strange occurrence. 

Brent,
  I don't use xt-60 on my batteries but I do use some in charging applications. Maybe there was not a good connection between the male and female sides and maybe there was arcing and they spot welded themselves together. there are certainly enough amps for that to happen. If it were me, I would not play around. I would replace the XT-60 on the speed control and on that battery. Why fool around, the connectors are cheap enough and I would not be putting any Band-Aids like grease or non conductive lube which might cause other issues.
 Will

Yes