If no maximum timestep is specified, LTSpice varies the timestep on-demand so as to smoothen out processor usage. This produces weird errors on the FFT and results in chopped off transients. For example, I made a saw oscillator that discharged a capacitor very fast once a threshold voltage was reached. I looked at the capacitor current and noticed that the discharges had seemingly random peaks that weren't constant. This fooled me into thinking my circuit was somehow not reliable. After changing the timestep to 100nS though, all transients were the same hight and the circuit performed perfectly. Also, if your circuit is oscillating and you can't get it to stop, try lowering the timestep. Sometimes oscillation can be triggered by a too high timestep by a simulator fault.