The Soviet space program actually ignored and passed
on a launch-window for a Venus-Mercury flyby mission
that opened on January, 05, 1969 and ended on February, 02,1969.

The Soviets did launch Venera-5 on January, 5th, 69 [I recall it to this day; I was in grade 7 then]
and five days later they sent Venera-6 on its way to Venus as well.

The Soviets new of the astrophysics work of a NASA scientist named Dr. Sturm who came up with
the concept of gravitational assists for spacecraft, but alas the Soviets bypassed an opportunity
to beat the Americans with a flyby of the planet Mercury in 1969, allowing the Americans to get there
first in March, 1974.

The Venera-5 and 6 spacecraft were also erronously designed because stubborn and foolish
Soviet scientists still believed in the incorrect theory that Venus had temperatures not much more that
500 F and atmospheric pressures not much greater than 20 Bars, inspite of the results obtained
by the JPL Mariner spacecraft....As a result, Soviet engineers, were compelled to minimize Venera
spacecraft reinforcing and minimize their cooling systems which resulted in the failure of those spacecraft
unnecessarily. They learned the hardway when it came to designing the Venera-7 spacecraft.

P.S. The Soviets/Russians never did get to visit Mercury, even during another, better launch-window in
August, 1970.