I-86 no longer exists in Connecticut; it was a numbering for part of Interstate 84. Never an original number for a highway, the Interstate 86 designation has also been applied to Idaho's former I-15W (decades ago) and New York's Southern Tier Expressway, NY 17 (on Dec. 3, 1999). It's the only 2-digit interstate number that has been signed on three separate routes.
In 1968, a new interstate link to Providence was approved. For a short time, the planned number was Interstate 82. I-82 would have continued easterly from I-84 near the East Hartford / Manchester border to Willimantic, Killingly, and Providence.
However, officials (probably on the Connecticut side) decided the I-82 number was unsuitable and instead proposed rerouting the eastern portion of I-84 onto the I-82 route. The leftover portion of I-84 (Manchester to Union, and onward to the Mass Pike) would get a new number: I-86. This was unofficial in early 1969 and made official by the FHWA on June 21, 1971.
I-86 absorbs I-491 (all on paper)
Interstate 86 was also extended over the proposed I-491 link through East Hartford and Glastonbury to I-91 in Wethersfield. This highway, for which public hearings were held in the late 1950s, was canceled in 1973.
Eastern I-84 cancellation: I-86 no longer needed
The portion of I-84 toward Providence ran into opposition in Rhode Island and was cancelled in 1983. Connecticut then renumbered I-86 back to I-84, with the I-86 numbering officially deleted on Dec. 12, 1984. There's more information on the I-84/86 switch on the I-84 page.
From its inception in 1971 to October 1, 1980, I-86 was co-signed with Route 15, the original number for the Wilbur Cross Highway, for its entire length.
I-86 never had an interchange with I-84: the junction in East Hartford was not complete until years after I-86 was removed. The transition from I-84 to I-86 was a sign on the Wilbur Cross Highway stating "I-84 ENDS / I-86 to Boston".
The Boston Globe was a little late catching the number change back to I-84. A November 1990 sports story starts like this: "The onslaught continues. The Celtics cruised up and down I-86 last night, stopping at the Civic Center long enough to vaporize the Heat."
Roger Katz writes that there was (in mid-1999) at least one sign the state missed in its renumbering: one in Manchester directing drivers to I-86 for Hartford and Boston. He says it sits about 50 feet from a road that was since rerouted, but is still visible to drivers. (However, I don't know anyone who knows precisely where it is.)
Apparently there was a short stretch of transition time in the early 1970s when I-86 was signed as both I-84 and I-86.
A few other route numbering changes in Connecticut that have also been reverted: