In informal language there are structures that aren't full sentences:
Lot of animals come into my garden. For example, birds.
The second part has be punctuated as a sentence and in some ways it expresses a new idea, but it does so without forming a sentence. In formal writing you might write:
... For example, there are birds / ... For example, birds come into my garden. / For example, birds do.
There is no main clause in "For example, birds." because there is no clause at all.
You example is more complex. There is a subordinating conjuction "when" but no matrix clause. There is no main clause, and this is a sentence fragment. These are avoided in formal writing, but are quite common when speaking informally.
We would understand it this subordinate clause to function as either a complement
Like, There was when I ...
or a subject
Like, when I put ... tank was something I certainly should not have done.