  We point out that in the case of a heavy top quark T, present in the Littlest
Higgs model (LH), and the t-T mixing parameter x_L > 0.90 the contribution to
B^0_{d,s}-\bar B^0_{d,s} mixing from box diagrams with two T exchanges cannot
be neglected. Although formally O(v^4/f^4) with v = 246 GeV and f > 1 TeV, this
contribution increases linearly with x_T = m_T^2/M^2_W and with x_T =
O(f^2/v^2) constitutes effectively an O(v^2/f^2) correction. For x_L ~ 1, this
contribution turns out to be more important than the genuine O(v^2/f^2)
corrections. In particular it is larger than the recently calculated O(v^2/f^2)
contribution of box diagrams with a single T exchange that increases only
logarithmically with x_T. For x_L = 0.95 and f/v = 5,10,15, the short distance
function S governing the B^0_{d,s}-\bar B^0_{d,s} mixing mass differences
\Delta M_{d,s} receives 56%, 15% and 7% enhancements relative to its Standard
Model (SM) value, implying a suppression of the CKM element V_td and an
enhancement of \Delta M_s. The short distance functions X and Y, relevant for
rare K and B decays, increase only logarithmically with x_T. With the
suppressed V_td, K->\pi\nu\bar\nu and B_d->\mu^+\mu^- decays are only
insignificantly modified with respect to the SM, while the branching ratio
Br(B_s->\mu^+\mu^-) receives 66%, 19% and 9% enhancements for x_L = 0.95 and
f/v = 5,10,15, respectively. Similar enhancement is found for
Br(B_s->\mu\bar\mu)/Br(B_d->\mu\bar\mu).
