It has been very difficult to deal with errors using Multi-User tools that sometimes could happen, due to some system malfunction. Particularly when trying to check-in objects and the error "Checked-Out object changed" triggers out! There is no way to identify which object/objects are responsible for this error.
As it is now, the only solution for this error is to save the slave file, open the master model and "Cancel Check Out", deleting the user where this error appearred. Re-building a new connection to master model...and do all work again.
Unfortunately this error seems to happen without a logical explanation...as all working flow procedures are being followed....the master model is never used (not even for an open / close), and even though sometimes when trying to check in, this error comes up...fortunately isn't on a daily basis and it shouldn't be happening ever!
It could help if a log file is created, indicating the "Handle Id's" of the objects that are causing this error, thus allowing to delete them on the slave model and allowing the check in operation to proceed.