Sometimes when I draw a line the 2 ends of the line connect with a straight thick line.

[gabriel_moose]

Here's a video: How do I fix this in Photoshop? - YouTube

[fenjas29325284]

Just my guess:

Don't let your finger touch the Shift key.

Fenja

[gabriel_moose]

I draw with a tablet.

[Theresa J]

Holding down your shift key between two clicks of the mouse will draw a straight line like shown in the video. Maybe your mouse is sticking, but I'm just guessing too. Try resetting your preferences.

[Trevor.Dennis]

Hey, cool trick.  Is there a plug-in for this?

It looks like you are using Windows 8.1 or 10, and Photoshop CC.  Is that right?

Does Thereas's remark about a possible modifier key make any sense?  Are you using a tablet, for instance, and touching thew rocker switch?  If you are using a tablet, which one?  I could see a tablet driver causing something like this.

If you are using a tablet and the brush preset is set to brush size by pen pressure, does the thick line correspond to what the mouse would draw?

Can you correlate the issue with particular circumstances?  How frequently does it happen, and does iot happen with other tools that use a brush like Healing Brush, eraser, clone tool etc.?  Can you test that for us if not sure please.