The EU Accounts -
https://fullfact.org/europe/did-audi...ign-eu-budget/
Prior to 2007 The EU’s Court of Auditors described the EU's budget as "not entirely" fair and accurate. Between 1994 and 2015 the Court described the payments as not being "free from material error" which is to say there were significant accounting errors.
I.e. money was being paid for things that didn't exist and the process itself was being managed in an unfair (corrupt) way.
Since 2007 the Court has described the accounts as fair and accurate but prior to 2016 (the year the UK voted to leave) there remained significant material inaccuracies.
The Accounts didn't get a clean bill of health in 2016, either, they were just "mostly" accurate.
https://www.eca.europa.eu/lists/ecad...ts-2017-en.pdf
The most recent report, for 2017, indicates that there continue to be material "errors" in reimbursement-based payments, though this was less than in 2016.
In essence - the EU pays for things people claim they are entitled to be reimbursed on without the attending paperwork.
This is, fundamentally, corruption.