Occasionally when a player places an acacia sapling and lets it grow or bone-meals it near an existing oak or dark oak tree (so that the acacia replaces some of the oak) the server crashes and restarts, generating a log.

I'm using pretty mainstream plugins and they aren't generating an exception prior to the crash.

Here is a crash report from one of the episodes:
http://pastebin.com/KGqzUyiL

Here is the last part of a log from the latest incident
http://pastebin.com/HsMJYL3s

I can't replicate it on a LAN server with all the same plugins and settings. it's very rare that it happens, but can happen a number of times in a row until saplings are destroyed (also hoppers randomly break when items thrown in, unsure if related)