If you get an error that looks like something like this:
"segfault at 0 ip b7955947 sp bfa35d70 error 4 in libsf_engine.so.0.0.0[b7953000+8000]"
When you start Snort after you have upgraded to 2.8.6 from 188.8.131.52 (or whatever)
This means you are running 184.108.40.206 SO rules with the 2.8.6 engine. You need the 2.8.6 rules to run with the 2.8.6 engine.
You can get the rules here: http://www.snort.org/snort-rules
Make sure you read this post too: http://blog.joelesler.net/2010/04/new-vrt-rulepack-changes.html