There was a notable hand on Ultimatebet in 2008 in which Phil Helmuth won a pot with a weaker hand.
You can see a replay of the hand here:
http://www.pokertableratings.com/hand-history/absolute/1162170993/418635327
According to the Ultimatebet CEO Paul Legget, the software malfunctioned and mistakenly awarded Helmuth the pot. After the incident was brought to attention of Ultimatebet, Phil had the money deducted from his account and the actual winner (DOUBLEBALLER) was awarded the money.
Mr. Legget said that the software error occurred because "the winning player disconnected from the table at the precise millisecond when the software was awarding the pot."
Ultimatebet has claimed that they promptly corrected the issue and it shouldn't happen again.
- Michael