I've done some collision analysis of the wikis at [[WikiPopularity]]. The results are at [Wiki Structural Markup Usage] and [[Wiki Character Markup Usage]]. (I've split this into two pages so that the tables display across the width of a single page.)

The analysis shows:

# the choice of {{{**}}}..{{{**}}} and {{{//}}}..{{{//}}} for bold and italic is collision free
# {{{{{...}} }}}for images collides with following wikis:
## [[MediaWiki]] for templates
## [[Confluence]] for mono
## [[JSP Wiki]] for mono
## [[Wikispaces]] for mono
## [[StikiPad]] for mono
# {{{{{{...}}} }}}collides with MediaWiki template parameters
# {{{==}}} for headings has two partial collisions:
## [[TikiWiki]] where {{{===...===}}} is used for underline
## [[DokuWiki]] where the heading order is reveresed, that is {{{=====}}} is a level 1 heading
# {{{[[link|text]]}}} has a partial collision with [[TiddlyWiki]]
# {{{*...}}} (bulleted list) has a collision with [[PukiWiki]] where it is used for heading
# {{{#...}}} (numbered list) has a collision with [[Midgard Wiki]] where it is used for heading
# {{{<<...>>}}} has a collision with [[TiddlyWiki]], where it is used for macros


**Please add the collisions with markup of wiki engines you know!**