When I left my first job they made me work the whole notice period even though I was going to a direct competitor.
On my last day I was kept so busy everyone else was down the pub for my farewell drink and I was still working!
Never mind, you live and learn.
When I left my next job they were advertising for my replacement even before I'd handed my notice in because they knew of the offer and knew I couldn't refuse it. That time i needed to stay long enough to get a new design reviewed while still, as an employee, protected by the 1977 Patents act.
Redundancy..... not a good experience for anyone but when this happened to me it was elective.
I was determined that if I went, rather than simply resign, I would go with some of their cash and had been waiting the opportunity.
Came the day and two of us were told our jobs were being made redundant and there was a one new position we could apply for. I said "thanks, but no thanks." (it would have served them well if we both said it but my colleague had a number wives to support (one current and one ex) and had no choice.
Next thing I know it has been temporarillly suspended so I could go to Russia and supervise a new installation. In their wisdom they had neglected the serious financial exposure a failed system could give them.
I made them pay for that too! (and some day hope to cash in the brownie points earned: probably when hell freezes over, but you never know)
JMW