One of the songs I play called Columbia, Your My Home, has a very difficult section involving "pinching" on the treble and bass strings followed by an offbeat treble note followed by another pinch alternating the bass in very rapid and syncopated 4/4 time. Here's the tab for two measures in G:
1------3M---------------------3M----------
2------------0I---3I----OI--------0I--3I--0I
3---------------------------------------------
4-----------------0T-------------------0T---
X---------------------------------------------
6------3T---------------------- 3T-----------
After the third repitition, rush a treble note on the on the first string, third fret a half beat. That's right, it's syncopated. Anyway that's about the most difficult riff I know in acoustic blues. I always have trouble with it. Well Yesterday I played a Santa Cruz rosewood dreadnaught ($3000) and I nailed it! Again and again! Damn, what a sweet ride that was!