We show that the tail probability of the rough line integral $$\int _{0}^{1}\phi (X_{t})dY_{t}$$
∫
0
1
ϕ
(
X
t
)
d
Y
t
, where (X, Y) is a 2D fractional Brownian motion with Hurst parameter $$H\in (1/4,1/2)$$
H
∈
(
1
/
4
,
1
/
2
)
and $$\phi $$
ϕ
is a $$C_{b}^{\infty }$$
C
b
∞
-function satisfying a mild non-degeneracy condition on its derivative, cannot decay faster than a $$\gamma $$
γ
-Weibull tail with any exponent $$\gamma >2H+1$$
γ
>
2
H
+
1
. In particular, this produces a simple class of examples of differential equations driven by fBM, whose solutions fail to have Gaussian tail even though the underlying vector fields are assumed to be of class $$C_{b}^{\infty }$$
C
b
∞
. This also demonstrates that the well-known upper tail estimate proved by Cass–Litterer–Lyons in 2013 is essentially sharp.