Mass update records via list view

Mass update opportunity records via a list view. Open the Enterprise Opportunities list view, filter to Close Date this quarter and Stage equals Negotiation, select all 120 visible records, change the Stage to Proposal and add a note in the Comments field: 'Stage progressed per pipeline review, contact client for revised close date.' Perform the update and confirm that 120 records changed, then notify the sales ops channel.

Before I mass-update stages on the Negotiation list, reduce the chance of mistakes: show Opportunity ID and Owner in the view, add a validation column that flags closed-stage mismatches, run a sample update on 10 records and report which owners were affected, and produce a CSV backup of the 120 records to the shared drive in case we need to revert.

I attempted a mass update of 120 negotiation-stage opportunities but it failed halfway and left about 40 records unchanged. I don't know whether this is a permissions issue, a validation rule, or a platform timeout. The reps expect a clean pipeline for tomorrow's forecast. What is the most likely cause and the immediate steps to finish the update safely?

I frequently have to revert mass updates because validation rules or missing fields block some records, and that creates distrust from reps. I waste hours backing up and redoing changes. What process or checklist should I adopt so mass updates succeed first time and I stop breaking the pipeline?