Two prime examples are:
BT employees, shares were worth £14 a share 1990's, most employees were dependent on this value being retained until retirement, look at the price now.
Lloyds TSB same principle as above, employees end up with tunnel vision and only focused on their companies stock, and don't sell up when things start to slide. I have seen a real life example of this in my family, shares we worth £5+ fell to -30p. 100K worth of shares now worth 20K.
Take advantage of discounted company share options but be prepared to sell when things don't look good, diversification is key to successful investment.