it's always the ones you least suspect: unis, page-8

Loogs, not all gaps (or windows in candlestick terms) are
meant to be filled, breakaway gaps in particular. (I'm not
much of a believer in gaps, with the exception that breakouts
on high volume accompanied by gaps outperform breakouts
without gaps... see Bulkowski's research).

Dual listed stocks create further complications. Obviously,
if market sensitive news is released whilst one exchange is
closed, gaps are far more common as there is a "catch up"
factor when the other exchange opens.