If it's any version of Slic3r beyond 0.7.2, it's quite likely a bug
Seriously, there hasn't been a bug-free version since 0.7.2.
I occasionally install the new ones and play with them, but always end up going back to that. The new ones are way too unpredictable and wonky. About the only bug 0.7.2 has, besides useless overhang support generation (which counts as a "feature", i guess), is that it creates a free-standing "dot" outside sharp corners, which is detached from the actual object and easily removed after printing.