The regular expression used for number highlighting (around line 2154 in geshi-1.0.X/src/geshi.php) is highly recursive. If the text to be highlighted has long stretches without other tokens, it is easy for this to exceed PCRE's recursion limit or to crash if PCRE's recursion limit is not low enough.
This is, fortunately, quite easy to fix for the common case. Instead of "(?!(?:<DOT>|(?>[^\<]))+>)", use "(?!(?:<DOT>|(?>[^\<]+))+>)" and you're good as long as there are not a huge number of "<DOT>" tokens.