In the UK, Lloyds TSB split into 2 banks (Lloyds and TSB) in 2013.
tl;dr
In April 2018 1.9 million TSB customers were left unable to bank online, some for several weeks; this was caused by customers being moved to a new system, which was only tested offline.
"Other failings by TSB that it identified included setting "unnecessary" time constraints, which did not understand the complexity of the project, and being dishonest about the reasons for delays."
The outage has cost TSB £370m.