On the 11th of November 2019, a transit of Mercury across the Sun will take place between 12:30 and 18:00UT. At this time, Mercury will be visible against the solar disc by obscuring a tiny part of it. After the 2019 event no further astronomical transit of Mercury will take place until December 2032.
Although this is true for the astronomical event, astrologically speaking, the symbolic conjunctions between Mercury and the Sun are by no means rare events. Nevertheless it might be of interest to take the opportunity and have a look at this particular conjunction.
The exact astrological conjunction will take place on the 11th of November 2019 at 15:22. The chart below, cast for London, shows the event.
The conjunction is placed in 18*55 Scorpio; Sun and Mercury are in a separating sextile to Saturn. With the greater malefic involved, it may be expected that this event will be of some significance.
Let us now have a closer look at Mercury. What immediately springs to mind is the fact that he is retrograde. To understand the significance of this, we may have a look at what traditional astrologers have to say about this.
Abu Ma’shar informs us in his Abbreviation of the Introduction to Astrology in the chapter on The Impotence of the Stars
“… but the worst [is] the retrogradation of Venus and Mercury, particularly if they are hidden by the Sun; or [planets are] found [under the rays of the Sun or] in dark degrees.” (Dykes, Introductions to Traditional Astrology, p220).
This immediately cuts to the heart of the matter of our investigation about Mercury conjunct Sun. When applying to the conjunct with the Sun, Mercury will not only be retrograde, he will as well be under the sunbeams and later on combust. That is exactly the worst-case scenario, highlighted by Abu Ma’shar in the quote above.
To understand what this means, we have to look at the definitions of these two terms.
- Firstly, a Planet is said to be combust when it is between 17.5 minutes and 8.5 degrees of the Sun.
- Secondly, under the Sunbeams is a harmful area, extending from the end of combustion until a Planet is 17.5 degrees apart from the Sun.
William Lilly informs us in his Christian Astrology, that Mercury
“is cold and dry, and therefore Melancholy; with the good he is good, with the evil Planets ill; … he is author of subtlety, tricks, deceives, perjury, &c.” (CA, p77)
And Mercury will be very much afflicted on the 11th of November. We have established so far that he will be retrograde, under the sunbeams and later on, closer to perfecting the conjunction, combust. Apart from that he will be aspecting Saturn, the greater malefic.
If we put all this information together, we come to the following conclusion.
Due to his natural and also his debilitated state during the conjunction to the Sun, Mercury will not affect us in a positive, measured manner but he will rather test some of us to a high degree. There can even be some level of fanaticism expected. It is also very likely that things will go wrong on an epic scale. Particularly electronic devices may unexpectedly malfunction, or the odd communication breakdown may occur. It is also not advisable to sign any kind of contract or agree to any binding arrangements during this transit.
Robson knows that this:
“hinders journeys and marriage, harvest and merchandise, favorable for redemption of captives.” [Robson, Fixed Stars and Constellations, p.72.]
It is likely that people who have Scorpio rising in their nativity or have their natal Sun or Mercury at or around 18* Scorpio will experience some of the effects of this conjunction.
The only time when the negative influence of the Sun will be turned into a more positive one is the time Mercury spends in the heart of the Sun, otherwise known as cazimi. This beneficial span of time, which is only very short, will be on 11th of November between 12:28UT and 18:16UT.
Peter Stockinger's Traditional Astrology Weblog 
What if someone is born with these COMBUSTIONs is there a positive explanation?
If you find that you have a combust planet in your birth chart, you will have to accept that whatever the planet signifies will be weakened.
As always, a good astrologer will take the whole of the chart into consideration, but will be able to tell you how this affliction is likely to affect you throughout your life.