I was jamming with Renoise and testing new drumsamples that I had recorded earlier.
I was adding some bassline to my song and I generated new sample using “Custom Wave Synth V0.90”. And after that the master scope in track scopes started to look like this:
2472 
When I played that sample from keyboard, it freezed that scope. And scope started moving again when button was released. Same time scope drawed some weird shapes (as in picture).
I generated sample using wavetype “Arc Sine” from C-4 to C-4. After restarting renoise everything seemed to work OK.
After some testing I noticed that this bug occurs only if I generate sample using “Arc Sine” or “Arc Cosine” wavetypes from C-4 or B-4.