A day doesn’t go by that someone isn’t asking a question in Slack #migration about how to troubleshoot a specific problem with a tricky migration. Almost universally, these problems be demystified by using Xdebug and putting breakpoints in two spots in Core’s MigrateExecutable. First is in the ::import() method where it rewinds the source and then processes it. The second place I regularly put a breakpoint is in ::processRow().