In some circumstances, it may even be violating your termination agreement with the former one.

Often, if made redundant by a company, you cannot return to work for them (as employee or contractor) for a year, otherwise you have to pay back your redundancy pay.

That leads to a potentially hilarious conversation about rates.

"I'll do it, my rates are 100 per hour plus 10K if you want me to start before June next year"