Until you fail to deliver something on time, employer gets angry and either fires you or worse finds out about you breaching contract and sues.
Then you would be paying through the nose and lawyering up. I know exactly of one such case only, though. I live in EU, and heard it was resolved by mediation but expensive for the party who breached it.
I heard about a few more cases where someone was caught illegally subcontracting. These also tend to be much cleaner.
If you do it smart they won’t be able to prove any wrongdoing. E.g. your second job should be contract work through your own offshore LLC, so you won’t show up in The Work Number and similar databases.
A different evasion tactic could be getting paid in crypto.
Then you would be paying through the nose and lawyering up. I know exactly of one such case only, though. I live in EU, and heard it was resolved by mediation but expensive for the party who breached it.
I heard about a few more cases where someone was caught illegally subcontracting. These also tend to be much cleaner.