The idea of extensions reserving any namespaces in the low-100's was an ill-advised move which should be abandoned as site admins have been told since the earliest MediaWiki versions (such as 1.4 and earlier, which had one-byte namespace numbering) that this was the range to use on the off-chance that MW's internal namespaces might expand into the 16+ range (they never have, the internal namespaces we had then we have now).
Extensions like DPLforum (which used to be installable on any manually-created namespace) are now going to $wgExtraNamespaces and registering some arbitrary number (110) themselves by automated means (as if they owned it outright) - even on sites where it is already in use. I just finished renumbering about a dozen wikis which had 'Forum:' elsewhere in the low-100's or on some number just above the MW built-in messages. A few had 110 already in use for something else, dropping visitors into the wrong namespace when they attempted to use the forums.
DPLforum was working perfectly well on all of these wikis since 1996 on some other namespace (various, MW 1.06 through 1.17, upgraded over the years) but the version currently in trunk (with MW 1.20) inserts 110=>Forum and if the original site had it somewhere else, things break.
The administrator installing these packages needs to have the ability to choose a namespace number (or at least not have one default to a number in the original site-specific range, with no check that those are already in use). Arbitrarily predefining one, especially one in the low-100's right in the middle of the range which had always been intended for site-specific namespaces, is asking for trouble.
DPLforum worked fine for years without forcing a selection of 110 as NS_FORUM. It should have been left that way.