Transactions T
1
and T
2
start at nearly the same time, and both read the balance as £100.
T
2
increases bal
x
by £100 to £200 and stores the update in the database. Meanwhile, transac-tion T
1
decrements its copy of bal
x
by £10 to £90 and stores this value in the database, over-writing the previous update, and thereby ‘losing’ the £100 previously added to the balance.