Example:

$queryBuilder

    ->andWhere("alias.field = :paramname")
    ->setParameter('paramname', $paramval)

    ->andWhere("alias.field = ?0")
    ->setParameter(0, $paramval)

;

If I call getParameters(), only 1 param is returned.

When executing the query, an exception is thrown:

Too few parameters: the query defines 2 parameters but you only bound 1

This seems to happen only if the param is named ?0.
With ?1, it seems to be ok.