Sine generator - Bubba Oscillator
You have created an ideal (infinite bandwidth OPAMP, infinite output voltage etc...)  model which is conditionally stable &amp; executed it. 

look at your y-axis, it has reach 1.5x\$10^{59}\$  HUGE!. This is what is causing an exception... its run out of resolution to represent this unstable "oscilator"



IF a closer to reality OPAMP is used (+-15V, gain limited to 100,000) it behaves