RCSI (Read Committed Snapshot Isolation) is something that makes databases go faster. But today I read a post by Michael J Swart that shows a much needed fix if you want to do ETL from an busy database with RCSI.

You need to use WITH(READCOMMITTEDLOCK), if your source table can get rows in concurrent batches.

I think both go into my toolbox for future use.

Find them at
https://erikdarling.com/sp_loghunter/ and
https://erikdarling.com/sp_healthparser/