Check the battery.

Anything is possible but what is most probable is that you need another thermometer since the outside temp where you are was 46F at 16:00 hrs or 4:00 pm. Not going to get bogged down into the physics of it all but when you have an ambient temp like that outside the inside temp could never reach what you mentioned in your greenhouse. In short check the battery if you have an electric thermometer.

Fahrenheit or Celsious? I find that our greenhouse temperatures rise to about one and a half time outside temp and can go to nearly double on a cold but sunny day. If you have a mercury or alchohol thermometer it is very likely correct.

99C =210F= The temp of a nice cup of tea. Again, check the battery.
99F=37C = The ambient temperature around my Piña Colada in Martinique. Check the battery.
Hank could have given a bit more information to get better answers such as the outside temp and time of day this was observed( I had to go to the web to get the current temp of Manchester) . Also as to the type of greenhouse cover, was it glass or plastic and finally, the type of thermometer used. When writing a question, it is very good to put yourself in the shoes of the one you are writing it to. Most likely, that person knows nothing unless you inform them and the less that person knows the more likely assumptions and guesses will be made in giving the answer.

Sorry, L, for the lack of info. I never thought to give chapter and verse which is typical of me. and I only tend to look at the temperature early morning and evening unless I'm in there during the day doing something which I wasn't yesterday. The greenhouse is glass and the floor is paving slabs if it helps.
It's a min and max thermometer which I've had for 5 years and have never seen it completely wrong before.
I have just phoned the manufacturers to see if they could add anything or give their opinion but they're not there until tomorrow. They say they will return my call. I wonder if they'll give me a replacement ?
Very doubtful I'd think. (£11.95 apparently )

Is it an LCD display, Hank? Usually those screw up if they're exposed to temperatures below zero centigrade (32 F), and certainly screw up if the temperature gets below -5 deg C and stays there for a few days.

I'm assuming you mean fahrenheit when you say 99 ... and actually, it's not impossible, if you had full sun all day like we did yesterday, and your greenhouse is in sun for a lot of the day, doors and windows closed. I was sitting in a friend's garden around 1.30 pm yesterday, outside, in a sheltered spot out of the wind, and we stripped down to teeshirts, lovely it was, positively hot. Chilly wind if you were in it though.

It was very sunny yesterday afternoon as I was driving (with the car windows open) on the M6 near Lancaster (had to root in the glove box for my cricket sunglasses). So, thermometer in greenhouse plus a bit of sunshine and 99 deg is quite possible.
I'm assuming that as it's a min/max thermometer it is one of the U shaped Mercury jobs (very reliable) with little sliders that mark the min/max (sliders can get stuck but usually move again with a couple of taps on the tube).