Notes this issue does not occur on package updates 1.3.1 and above, but does occur on packages 1.2.4 and below. 
# Create a new Unity project
# Open window> Package Manager and then change from 'In Project' to 'Unity Registry'
# Search or Find the collections package and then select 'Install'
# In file explorer find the Project's 'Manifest.json' file and open it in a text editor or IDE
# Change line "com.unity.collections": "2.1.4", from '2.1.4' to '1.2.4'
# Switch back to the Editor and observe that the collections version number has changed
# Select 'Update to 2.1.4'

Expected result: the package is updated and no errors are logged

Actual Result: 'UnsafeParallelHashSetExtensions.gen.cs' is thrown 243 times in the console and the player is unable to enter playmode or clear these errors unless the project is reopened.