Aspects of GCSE English - a core subject taken by more than 700,000 students a year - have become easier recently, the exams regulator says.
Exams vary between boards and are constantly changing
An expert review of standards for the Qualifications and Curriculum Authority said booklets given in advance to students made questions "predictable".
It also found some exam boards' test papers were less demanding than others.
The review covered exams set by the five main boards in England, Wales and Northern Ireland.
Schools in each nation can and do "mix and match" qualifications from different boards.
Among other subjects, a study of music over 20 years found a "significant loss of demand" in some areas.
In English, the panel compared standards in 2002 and 2005 and felt most changes had made courses clearer and more effective, and had reduced variations between the boards without changing demand overall.
But releasing reading material to students prior to the exams, whether in anthologies or in pre-release booklets, "had created an imbalance between foreseeable and unforeseeable demand".
This was compounded by "the predictability of many writing and reading tasks set in examinations".
The problem was there in 2002 but had become more noticeable in 2005.
"Reviewers judged that in some cases this left insufficient unforeseeable challenges," the QCA report said.
"This was of particular concern given that GCSE English is a core subject and an often essential gateway to progression in education/training or entry to a career path."
It added: "Overall, the reviewers judged that the Edexcel scheme of assessment was less demanding than those of the other awarding bodies."
Different boards allowed different amounts of time to answer question papers.
OCR was felt to be toughest in this respect, while CCEA was "undemanding" compared to the others.
But the OCR board's GCSE had been made modular, allowing candidates to take units several times, with only the best mark counting.
"In addition, candidates could take the same unit in the same series both as coursework and as a written examination.
"Reviewers judged that this marked a reduction in demand, making the scheme easier than the other awarding bodies in this respect."
In music, a review spanning two decades from 1985 looked at both GCSEs and A-levels.
It found GCSEs were more "accessible" and now featured jazz, pop and world music alongside the Western classical tradition.
A compulsory requirement for performance and composition meant students now had to have firsthand musical experience, and could be more creative.
But the minimum demand of pieces needed to achieve higher grades had become "very low", the reviewers said.
Coursework had replaced live concerts while shorter, multiple-choice questions meant candidates could not develop essay-based answers requiring detailed knowledge of set works.
At A-level, several areas were less demanding "meaning that some important skills and understanding are not being as fully developed as they were for candidates in the past".
There was no longer the same impetus for the most able performers to challenge themselves.
And again there was less need for extended written work.
QCA chief executive Ken Boston acknowledged there were some "issues" around maintaining standards and promised "continued vigilance" so people could have confidence in the qualifications.
A spokesman added: "Last year new criteria were published for A-levels which will address some of the issues that have been raised in these reports, including less formulaic questions and greater opportunity for writing extended answers.
"Later this year new criteria will be published for GCSEs."