https://understanddrupal.com/articles/how-debug-drupal-migrations-part-1

explode_missing_delimiter.yml

# Source values: 3.1415, 2.7182, and 1.4142

psf_number_components:
  plugin: explode
  source: src_decimal_number

explode_missing_delimiter_messages.txt

 ------------------- ------- --------------------
  Source IDs Hash    Level   Message             
 ------------------- ------- --------------------
  7ad742e...732e755   1       delimiter is empty 
  2d3ec2b...5e53703   1       delimiter is empty 
  12a042f...1432a5f   1       delimiter is empty 
 ------------------------------------------------

explode_missing_delimiter_output.sh

$ drush mim ud_migrations_debug
[notice] Processed 3 items (0 created, 0 updated, 3 failed, 0 ignored) - done with 'ud_migrations_debug'

In MigrateToolsCommands.php line 811:
ud_migrations_debug Migration - 3 failed.

explode_with_log.yml

psf_number_components:
  - plugin: explode
    source: src_decimal_number
    delimiter: '.'
    strict: false
  - plugin: log

explode_with_log_messages.yml

 ------------------- ------- --------
  Source IDs Hash    Level   Message 
 ------------------- ------- --------
  7ad742e...732e755   1       3 
  7ad742e...732e755   1       1415
  2d3ec2b...5e53703   1       2
  2d3ec2b...5e53703   1       7182
  12a042f...1432a5f   1       1
  2d3ec2b...5e53703   1       4142
 ------------------------------------

explode_wrong_type.yml

psf_number_components:
  plugin: explode
  source: src_decimal_number
  delimiter: '.'

explode_wrong_type_messages.txt

 ------------------- ------- ------------------------------------
  Source IDs Hash    Level   Message                             
 ------------------- ------- ------------------------------------
  7ad742e...732e755   1       3.1415000000000002 is not a string 
  2d3ec2b...5e53703   1       2.7181999999999999 is not a string 
  12a042f...1432a5f   1       1.4141999999999999 is not a string 
 ----------------------------------------------------------------