When viewing the Wikipedia article https://en.wikipedia.org/wiki/Arithmetic_group in the iOS Wikipedia app, there is displayed just above the section heading "S-arithmetic groups" a string that reads (as a top level heading):
The source for the heading is
== <math>S</math>-arithmetic groups ==
I presume this is some sort of error message. The problem is fully reproducible on my device. I am using an iPhone X running iOS 11.3 and there are no pending updates. The problem does not occur when viewing the same article in Safari on my iPhone nor in Safari on my MacBook.