There are ways for a straight Wizard character (for example, via Spontaneous Divination ACF or Uncanny Forethought feat) to get some spontaneous spellcasting capability and enter an Ultimate Magus PrC without belonging to another spontaneously casting class.
An Ultimate Magus' description states:
Spellcasting: At each level except 1st, 4th, and 7th, you gain new spells per day and an increase in caster level (and spells known, if applicable) as if you had also gained a level in both a prepared arcane casting class and a spontaneous arcane casting class to which you belonged before adding the prestige class level.
For a straight Wizard entry, both 'prepared arcane casting class' and 'spontaneous arcane casting class' are the Wizard. Does it allow you, RAW-wise, to have a two-level advancement of your Wizard spellcasting capability in one character level? Can such an advancement be possible with some other PrC?