Staff numbers at the city's libraries has been reduced from 156 to 79 under £123 million cuts - leaving gaping shortages.
When the remaining staff took their booked off holiday over the past few weeks, it was discovered there was not enough cover to keep some of the libraries open.
On other occasions, staff sickness have also led to forced closures.
Council libraries chief councillor Elias Mattu, said: "Unfortunately there have been a few occasions over the last few weeks when we've had to close branches because of employee absence, and I'd like to apologise for any inconvenience this may have caused customers.
"This occurred primarily as a result of sickness or because employees had already booked annual leave prior to the restructure and we were unable to make alternative arrangements. The situation should stabilise over the coming weeks because the booking of annual leave is now coordinated across the library service which means that, where required, we can bring in cover from other libraries."
Reduced opening hours were introduced in April 2014 as part of the city council's programme of cuts.
The number of hours libraries are staffed has been reduced 656 hours per week to 421.
The main Central Library, which accounts for around half of all library visits in Wolverhampton, continues to be open 56 hours per week.
Bilston, Warstones and Wednesfield libraries are now staffed for 35 hours per week, Finchfield and Tettenhall libraries 30 hours per week and Ashmore Park, Pendeford and Penn libraries 24 hours per week. Long Knowle, East Park, Low Hill, Spring Vale and Whitmore Reans libraries are staffed for 15 hours per week.
Blakenhall Library, which is part of Blakenhall Community and Healthy Living Centre, is now self-service and is open at the same time as the centre. Collingwood Library at Broadway Gardens remains self-service and continues to operate for 15 hours per week.
Liberal Democrat campaigner Tom Stokes said it showed the council's scheme was flawed.
He said: "Yet again, this Labour council has made a mess and will no doubt blame someone else for their mistake. They always blame anyone and everyone other than themselves when it all hits the fan!"
Councillor Mattu added: "The city council is facing an unprecedented financial challenge as a result of central government cutbacks and must save some £123m by 2018-19 – on top of the £100m it has already had to save over the last few years.
"By reducing library opening hours, we've been able to make vital savings while also keeping our libraries open at the times when most people want to use them – and I am sure that customers would agree it is better to have access to libraries with reduced opening hours than to lose them altogether."