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Jonathan SpinkJonathan Spink 

Has anyone experienced deployment issues with the Summer '20 upgrade?

We have seen this error since the upgrade which has then suddenly disappeared:
"System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out"
Best Answer chosen by Jonathan Spink
SwethaSwetha (Salesforce Developers) 
HI Jonathan,
The issue you are facing is related to Summer'20 and has been identified as a bug by salesforce product team.

Please see Known Issue Link: https://success.salesforce.com/issues_view?id=a1p3A0000003UWIQA2

Salesforce had to revert the fix in Summer20 due to unexpected impact, and currently reviewing it for another attempt, with no current ETA. Please click on "This issue affects me" button of known issue to get notifications of on going progress.

Also see https://success.salesforce.com/issues_view?id=a1p3A000000ATF1QAO

https://salesforce.stackexchange.com/questions/307988/salesforce-2020-release-has-blocked-all-apex-tests-that-use-http-mocking
Hope this helps you. Please mark this answer as best so that others facing the same issue will find this information useful. Thank you

All Answers

SwethaSwetha (Salesforce Developers) 
HI Jonathan,
The issue you are facing is related to Summer'20 and has been identified as a bug by salesforce product team.

Please see Known Issue Link: https://success.salesforce.com/issues_view?id=a1p3A0000003UWIQA2

Salesforce had to revert the fix in Summer20 due to unexpected impact, and currently reviewing it for another attempt, with no current ETA. Please click on "This issue affects me" button of known issue to get notifications of on going progress.

Also see https://success.salesforce.com/issues_view?id=a1p3A000000ATF1QAO

https://salesforce.stackexchange.com/questions/307988/salesforce-2020-release-has-blocked-all-apex-tests-that-use-http-mocking
Hope this helps you. Please mark this answer as best so that others facing the same issue will find this information useful. Thank you
This was selected as the best answer
Jonathan SpinkJonathan Spink
Thanks Swetha.