The cases of three patients whose deaths could have been avoided are among more than 100 examples in a report on public service failures and poor complaint handling in the NHS in England and UK Government departments.
The report by the Parliamentary and Health Service Ombudsman contains 121 summaries of complaints investigated between December and January.
Set up by Parliament to provide an independent complaint handling service, the organisation made final decisions on 556 complaints during the period, of which 201 were upheld or partially upheld.
Individual examples included seven upheld or partially upheld cases of poor maternity care, as well as nine asylum seekers who waited years for a decision on their applications. Repeated instances of inadequate end-of-life care are also laid bare in the report.
The majority of the summaries are of cases upheld or partly upheld. The report said: "These are the cases which provide clear and valuable lessons for public services by showing what needs changing so that similar mistakes can be avoided in future.
"They include complaints about failures to spot serious illnesses and mistakes by government departments that caused financial hardship."
The organisation investigated 58 cases of avoidable death during the two-month period and upheld or partially upheld 29.
They included a man with learning disabilities in the West Midlands who died of multiple organ failure "after a series of failures in care and a lack of consideration for his rights as a disabled person".
Of the nine immigration cases in the report, it found most could have been avoided if UK Visas and Immigration (UKVI) communicated more effectively with applicants, responded to letters and kept better records.
In one example, a woman waited six years from 2008 to 2014 for a decision on her asylum application before she was granted leave to remain in the UK.
Ombudsman Dame Julie Mellor said: "Often people complain to us because they don't want someone else to go through what they or their loved one went through. This report shows the types of unresolved complaints we receive and the human cost of that poor service and complaint handling.
"Many of the complaints that come to us should have been resolved by the organisation complained about.
"Complaints provide an opportunity for learning and improvements and should be embraced at all levels of the organisation from the board to the front line."
Around 80% of the organisation's investigations related to the NHS in England, while 20% concerned UK government departments and their agencies.
Which? executive director, Richard Lloyd, said: "We know people are reluctant to complain because they don't feel confident that action will be taken.
"The Government needs to reform the broken complaints system in public services to prevent serious failings."