the system is full of bugs which should have been fixed but haven't - presumably because there isn't time to "slip" the implementation date as those charged to bring it in will lose their fat performance related bonuses.
for instance, problems typically affect claimants whose wages are paid on or close to the first day of their universal credit monthly assessment period, which is used to calculate how much a claimant is entitled to be paid each month. If a claimant is paid wages a day or two earlier – ie because their normal payday would fall on a weekend or bank holiday (for example; it happens to me, payday is the 28th but could be 26th if the 28th is on a sunday) – the system records them as having had two pay cheques in one assessment period and none in the following one.
As a result, universal credit considers them to be earning more than they actually do in the first period, so automatically blocks “passported” entitlements such as free prescriptions – and in the second period considers them to be earning nothing, so makes them subject to benefit-cap penalties.