The claim that 297 years of the Middle Ages were fabricated, and it is secretly the year 1725.
Interesting claims
Three invented centuries
Illig argued that Holy Roman Emperor Otto III and Pope Sylvester II backdated the calendar to place themselves in the year 1000, fabricating 614-911 AD, including Charlemagne. Evidence offered: suspicious gaps in archaeology and the calendar's drift.
Assessment: Collapsed under independent dating: tree-ring sequences, ice cores, astronomical records like Halley's Comet sightings in 837 and 760, and simultaneous histories from China, Byzantium, and the Islamic world that never got the memo.
Documented
The real calendar problem underneath
The Julian-to-Gregorian correction and genuine medieval dating inconsistencies do exist, and chronology was once genuinely loose. The theory feeds on a real weakness in how we assume the past was counted.
Assessment: Wrong conclusion, interesting question. Auditing the calendar is legitimate; deleting Charlemagne is not.
Stories
The emperor who wanted to live in the year 1000
The popular retelling: Otto III and Pope Sylvester II conspired to jump the calendar so that Otto would rule at the millennium. Charlemagne was forged wholesale, three centuries of monks backfilled the chronicles, and the paperwork of Europe is one enormous palimpsest.
Assessment: The story form of Illig's thesis, told in bars and forums with the same relish as the academic version, minus the footnotes. It is the most entertaining heresy in medieval studies, which is exactly why its few genuine anomalies never get sober hearings.
Sources and further reading
Heribert Illig, Das erfundene Mittelalter, Econ, 1996
Worth asking
If you learned tomorrow that three centuries were invented, what evidence in your own hands could prove them wrong?