plot(t) is in this case the same as
plot(t[[1]], t[[2]])
As the error message says, x and y differ in length and that is because you plot a list with length 4 against 1:
> length(t)
[1] 4
> length(1)
[1] 1
In your second example you plot a list with elements named x and y, both vectors of length 2,
so plot plots these two vectors.
Edit:
If you want to plot lines use
plot(t, type="l")