Tag algorithm breaks on some dangling tags

The following input is invalid.

[i]Emphasized and [b]strongly[/i] emphasized text.[/b]

As the parser uses a strict model (rather than guessing), it will be rendered as the following.

Emphasized and [b]strongly emphasized text.[/b]

However, the following will cause a fatal error:

[b]Strong[/b] and [i]Emphasized and [b]strongly[/i] emphasized text.[/b]

The correctly closed [b] tag in the beginning increments the open-tag counter without decrementing, causing the parser to believe the final [/b] tag is not dangling.