As promised:

UPDATE 29/03/2013: SMS delivery issues resolved with a workaround.

https://github.com/fairwaves/openbts-2.8/commit/c617e53a2c2eccfdf458021a981fbe94b9d488dc

We were recently looking into a bug when MT-SMS delivery was  interrupted with an unexpected UA frame on L2 layer. We’ve fixed this in our branch by ignoring a second UA in Established L2 state [1], but  the real issue lies in the filler table coupled with delays in  processing in higher layers.