Bourbaki dangerous bend symbol
Certains passages sont destinés à prémunir le lecteur contre des erreurs graves, où il risquerait de tomber; ces passages sont signalés en marge par le signe ☡ («tournant dangereux»)
Some passages are designed to guard the reader against serious errors, where he risks falling; these passages are signposted in the margin by the sign ☡ ("dangerous bend")
Nicholas Bourbaki's description of the symbol in several textbooks
The dangerous bend symbol ☡ (unicode number x02621) was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument. Others, including computer scientist Donald Knuth, used variations of the symbol in their textbooks.
Typography
In the LaTeX typesetting system, the dangerous bend symbol can be written as
\begin{danger} ... \end{danger}
\begin{ddanger} ... \end{ddanger}.This requires LaTeX3. [1].