"Methuselah trusts" are (very) longterm trusts that accumulate so much compound interest over hundreds of years that by the time they mature, they could potentially pose a real threat to the economy. That's why they're illegal. From Lapham's Quarterly:
Hartwick College didn’t really mean to annihilate the U.S. economy. A small liberal-arts school in the Catskills, Hartwick is the kind of sleepy institution that local worthies were in the habit of founding back in the 1790s; it counts a former ambassador to Belize among its more prominent alumni, and placidly reclines in its berth as the number-174-ranked liberal-arts college in the country. But along with charming buildings and a spring-fed lake, the college once possessed a rather more unusual feature: a slumbering giant of compound interest.
With bank rates currently bottomed out, it’s hard to imagine compound interest raising anyone much of a fortune these days. A hundred-dollar account at 5 percent in simple interest doggedly adds five bucks each year: you have $105 after one year, $110 after two, and so on. With compound interest, that interest itself get rolled into the principal and earns interest atop interest: with annual compounding, after one year you have $105, after two you have $110.25. Granted, the extra quarter isn’t much; mathematically, compound interest is a pretty modest-looking exponential function.
Modest, that is, at first. Because thanks to an eccentric New York lawyer in the 1930s, this college in a corner of the Catskills inherited a thousand-year trust that would not mature until the year 2936: a gift whose accumulated compound interest, the New York Times reported in 1961, “could ultimately shatter the nation’s financial structure.” The mossy stone walls and ivy-covered brickwork of Hartwick College were a ticking time-bomb of compounding interest—a very, very slowly ticking time bomb.