Pinyin or fully Hanyu Pinyin is the standard Romanisation for Mandarin. It is widely used and only in few cases older Romanisations like Wade-Giles or Bopomofo prevail. So as Pinyin is ubiquitous you think you've seen it all? Do you know characters ê, ẑ, ĉ, ŝ and ŋ? I didn't when I came across them some while ago; I believed them to be some kind of ad-hoc invention or imprecise form used by an uninformed author. But far from that. ISO 7098, the ISO form of Pinyin, lists this character (only as a final, in its single form still rendered e, but nevertheless) and others do, too. It's actually an interjection and used for 欸 by some sources.

ẑ, ĉ, ŝ and ŋ have a different use-case though. They seem to be a relict of the Pinjin draft scheme from 1956. Here initials zh, ch, sh and final part ng are expressed using only one letter, the latter as ŋ (there it is) and the other three as ᶎ (U+1D8E, LATIN SMALL LETTER Z WITH PALATAL HOOK; or this: ȥ U+0225, LATIN SMALL LETTER Z WITH HOOK), ɕ (U+0255, LATIN SMALL LETTER C WITH CURL) and ʂ (U+1D8A, LATIN SMALL LETTER S WITH PALATAL HOOK). Those characters, hard to find on any keyboard, probably were substituted with their -h counterparts later, still allowing a short form using the circumflex: Nǐ ẑīdao mā?

Thinking about that, you could believe that the ingenious inventors only anticipated the Taiwanese accent, making it easy to omit any retroflex pronunciation.